fix(eq): entregar los decibelios que pide el usuario, sin estirarlos

La app promete decibelios en cuatro sitios y no los entregaba en ninguno. El
slider abarca un +/-12 fijo, escribe el numero con su unidad debajo de cada
banda, y `equalizerBandValue` le dice literalmente "decibelios" a TalkBack. El
modelo documenta las bandas como dB y los presets de fabrica estan escritos en
dB. just_audio documenta `setGain` en decibelios y multiplica por 1000 para
llegar a milibelios sin normalizar nada: `minDecibels`/`maxDecibels` son la
CAPACIDAD del dispositivo, no una escala a la que normalizar.

Pese a eso, la ganancia se estiraba por `maxDecibels/12`. Un +6 dB llegaba como
+10 en un movil de rango ancho.

El estiramiento nunca fue una decision de diseño

Antes de a9202c6 el codigo era `setGain(preset.bandas[i])`, decibelios
literales. Ese commit metio la normalizacion sin docstring, sin test y sin nota
de diseño, y traia un fallo: 0 dB caia en el punto medio del rango, asi que un
preset PLANO realzaba. 3449e2c corrigio exactamente eso y nada mas -- su propio
mensaje dice que el objetivo era "0 dB es siempre 0" -- heredando el
estiramiento sin discutirlo. No hay ADR, spec ni comentario que lo justifique.

Lo que de verdad rompia: la portabilidad

Lo que se persiste y se exporta son los dB del usuario, sin escalar; el escalado
ocurre solo al escribir en el efecto nativo. Asi que el mismo backup suena
distinto en cada telefono, y la interfaz informa de una restauracion perfecta
mientras el audio no lo es. Peor con el rango asimetrico habitual de Android
([-12, +19]): los realces se multiplican por 1.58 y los cortes por 1.0, de modo
que el preset no solo sube de nivel, CAMBIA DE FORMA. Jazz [3, -1, -1.5, 2, 4]
se entregaba como [4.75, -1, -1.5, 3.17, 6.33]. Con presets por dispositivo, el
mismo preset se deformaba distinto en el altavoz y en el Bluetooth.

No es un clamp a secas

`db.clamp(minDecibels, maxDecibels)` habria reintroducido el fallo de 3449e2c:
en un dispositivo que reporte [+3, +19], el cero se convierte en +3 y el preset
plano vuelve a realzar. La ventana se fuerza a contener el cero, asi que se
conservan todas las invariantes ganadas -- 0 siempre es 0, el signo nunca se
invierte, el resultado nunca escapa del rango nativo, un dispositivo sin margen
en un lado no puede realzar por ese lado -- y solo desaparece el estiramiento.

Que se oye distinto: en un movil de +/-12 dB, identico a hoy. En uno de rango
ancho los realces bajan, hasta un 40% menos en dB en uno de +/-20. Los cortes
apenas se mueven, porque la forma habitual es [-12, +N] y el lado negativo ya
iba practicamente 1:1. A cambio, los seis presets de fabrica suenan por fin
igual en cualquier telefono.

Ningun preset guardado necesita migracion: lo almacenado siempre fueron los dB
del usuario.

Suite completa: 1534 pasan, 2 omitidos. flutter analyze mantiene los 5 avisos
preexistentes.

test/servicios/servicio_audio_eq_ganancia_test.dart

@@ -1,28 +1,43 @@
 import 'package:flutter_test/flutter_test.dart';
 import 'package:pluriwave/servicios/servicio_audio.dart';
 
-/// eq-estado-unico item E — `mapearGananciaNativa`, the translation from the
-/// app's fixed ±12 dB slider scale to whatever range the device's native
-/// `Equalizer.getBandLevelRange()` reports.
+/// `mapearGananciaNativa` — the hand-off from the app's ±12 dB slider to the
+/// device's native `Equalizer`, whose capability is reported as
+/// `AndroidEqualizerParameters.min/maxDecibels`
+/// (`Equalizer.getBandLevelRange()` in millibels, divided by 1000).
 ///
-/// This is the only source-plausible explanation for the reported «suena muy
-/// alto» half of the bug. The original implementation normalised the input
-/// across the WHOLE range and mapped it linearly:
+/// WHY THIS CONTRACT CHANGED — the previous one stretched each side of the
+/// slider against its own end of the native range, so `+6` on a device
+/// reporting `[-12, +20]` was delivered as `+10`. Both sides of the mapping
+/// are already the SAME unit, so that multiplication was a unit error:
 ///
-///     normalizado = (db.clamp(-12, 12) + 12) / 24
-///     return minDecibels + normalizado * (maxDecibels - minDecibels)
+///   * `just_audio` documents `setGain` as "Sets the gain for this band in
+///     decibels", and its Android bridge does `setBandLevel(band,
+///     round(gain * 1000.0))` — plain dB to millibels, no normalisation.
+///     `min/maxDecibels` are the device's absolute CAPABILITY in dB, i.e. a
+///     bound on the control, not a scale to normalise into.
+///   * The app makes the user a decibel promise in three places at once: the
+///     slider is hard-coded `min: -12.0, max: 12.0`, the label under each
+///     band prints `'${banda.toStringAsFixed(1)}dB'`, and TalkBack reads out
+///     `equalizerBandValue` = "{value} decibels". Stretching made that label
+///     a lie on every device whose range is not exactly ±12.
+///   * Presets are persisted and EXPORTED as those same raw slider dB
+///     (`PresetEcualizador.toJson`), so under the old mapping a backup
+///     restored on a wider-range phone showed identical numbers and played
+///     louder — and on the common asymmetric shape `[-12, +19]` boosts were
+///     multiplied by 1.58 while cuts were not, deforming the preset's SHAPE
+///     rather than merely its depth.
 ///
-/// which sends 0 dB to the MIDPOINT of the native range. That is only 0 when
-/// the range happens to be symmetric. Android does not guarantee that: the
-/// AudioEffect Equalizer contract only requires a min/max pair, and real
-/// devices ship asymmetric ranges. On such a device a FLAT preset — every
-/// band 0 dB — was silently pushing a positive boost into every band, which
-/// is audibly louder while the on/off button still reads "off".
+/// So: the number the user reads is the number the device is asked for. The
+/// native range only CLAMPS it.
 ///
-/// The contract asserted here: 0 dB always maps to exactly 0, and the two
-/// sides of the scale are stretched INDEPENDENTLY against their own end of
-/// the native range, so the sign of the user's intent is never inverted and
-/// the extremes still reach the device's real limits.
+/// What this deliberately KEEPS from the previous contract — every invariant
+/// the «suena muy alto» fix actually earned. 0 dB is always exactly 0 (a
+/// naive `db.clamp(minDecibels, maxDecibels)` would regress that on a wholly
+/// positive reported range, turning a FLAT preset into a boost again), the
+/// sign of the user's intent is never inverted, the result never escapes the
+/// native range, a device with no headroom above unity can never boost, and a
+/// zero-width range collapses to 0.
 void main() {
   group('mapearGananciaNativa — 0 dB is always exactly 0', () {
     test('symmetric range (the common case) is unchanged', () {
@@ -43,6 +58,10 @@ void main() {
     });
 
     test('a wholly positive range still cannot boost a FLAT preset', () {
+      // This is precisely why the mapping cannot be a plain
+      // `db.clamp(minDecibels, maxDecibels)`: that would answer +3 here and
+      // bring the «suena muy alto» bug straight back. The clamp window has
+      // to be widened so that it always contains 0.
       expect(mapearGananciaNativa(0, minDecibels: 3, maxDecibels: 19), 0);
     });
 
@@ -51,36 +70,61 @@ void main() {
     });
   });
 
-  group('mapearGananciaNativa — the extremes reach the native limits', () {
-    test('+12 dB maps to the native maximum', () {
-      expect(mapearGananciaNativa(12, minDecibels: -12, maxDecibels: 19), 19);
+  group('mapearGananciaNativa — the slider dB reach the device literally', () {
+    test('+12 dB is delivered as +12 dB, not stretched to the native max', () {
+      // CONTRACT CHANGE: this used to assert 19, i.e. the whole of the
+      // device's headroom. The slider says "12.0dB" and the accessibility
+      // label says "12.0 decibels", so 12 dB is what the device must be
+      // asked for. The 7 dB of extra hardware headroom is unreachable by
+      // design until the slider itself is widened and says so.
+      expect(mapearGananciaNativa(12, minDecibels: -12, maxDecibels: 19), 12);
     });
 
-    test('-12 dB maps to the native minimum', () {
+    test('-12 dB is delivered as -12 dB', () {
       expect(mapearGananciaNativa(-12, minDecibels: -12, maxDecibels: 19), -12);
     });
 
-    test('values beyond the slider scale are clamped, not extrapolated', () {
-      expect(mapearGananciaNativa(40, minDecibels: -15, maxDecibels: 15), 15);
-      expect(mapearGananciaNativa(-40, minDecibels: -15, maxDecibels: 15), -15);
+    test('values beyond the slider scale clamp to the slider limit', () {
+      // CONTRACT CHANGE: these used to answer the NATIVE extremes (±15).
+      // The slider scale is the first bound; the device range is the second.
+      expect(mapearGananciaNativa(40, minDecibels: -15, maxDecibels: 15), 12);
+      expect(mapearGananciaNativa(-40, minDecibels: -15, maxDecibels: 15), -12);
     });
   });
 
-  group('mapearGananciaNativa — each side scales against its own end', () {
-    test('half boost is half of the positive headroom', () {
+  group('mapearGananciaNativa — the label is the value the device gets', () {
+    test('+6 dB on a wide-range device is +6 dB, never 10', () {
+      // CONTRACT CHANGE: this used to assert closeTo(10) — "half boost is
+      // half of the positive headroom". A slider reading "6.0dB" that
+      // produced +10 dB of real boost is exactly what made a restored backup
+      // sound different on a different phone.
       expect(
         mapearGananciaNativa(6, minDecibels: -12, maxDecibels: 20),
-        closeTo(10, 1e-9),
+        closeTo(6, 1e-9),
       );
     });
 
-    test('half cut is half of the negative headroom', () {
+    test('-6 dB on that same device is -6 dB', () {
       expect(
         mapearGananciaNativa(-6, minDecibels: -12, maxDecibels: 20),
         closeTo(-6, 1e-9),
       );
     });
 
+    test('the six factory presets keep their shape on an asymmetric device', () {
+      // Jazz, authored in true dB before any scaling existed. Under the old
+      // mapping [-12, +19] delivered it as [4.75, -1, -1.5, 3.17, 6.33]: a
+      // different tonal curve, not merely a louder one.
+      const jazz = [3.0, -1.0, -1.5, 2.0, 4.0];
+      final entregado = jazz
+          .map(
+            (db) =>
+                mapearGananciaNativa(db, minDecibels: -12, maxDecibels: 19),
+          )
+          .toList();
+      expect(entregado, jazz);
+    });
+
     test('the sign of the user intent is never inverted', () {
       for (final db in [-12.0, -6.0, -1.0, 1.0, 6.0, 12.0]) {
         final nativo = mapearGananciaNativa(
@@ -97,12 +141,44 @@ void main() {
     });
   });
 
+  group('mapearGananciaNativa — a device narrower than the slider', () {
+    test('a request that fits is still delivered literally', () {
+      // CONTRACT CHANGE: the old mapping shrank this to (3/12)*6 = 1.5 dB,
+      // so a modest device silently under-delivered every request too.
+      expect(mapearGananciaNativa(3, minDecibels: -6, maxDecibels: 6), 3);
+      expect(mapearGananciaNativa(-3, minDecibels: -6, maxDecibels: 6), -3);
+    });
+
+    test('a request beyond the device range clamps to the device limit', () {
+      expect(mapearGananciaNativa(12, minDecibels: -6, maxDecibels: 6), 6);
+      expect(mapearGananciaNativa(-12, minDecibels: -6, maxDecibels: 6), -6);
+    });
+
+    test('a very narrow device still gets a sane, in-range value', () {
+      for (final db in [-12.0, -5.0, 0.0, 5.0, 12.0]) {
+        final nativo = mapearGananciaNativa(
+          db,
+          minDecibels: -1.5,
+          maxDecibels: 1.5,
+        );
+        expect(nativo, greaterThanOrEqualTo(-1.5));
+        expect(nativo, lessThanOrEqualTo(1.5));
+        expect(nativo.sign, db.sign);
+      }
+    });
+  });
+
   group('mapearGananciaNativa — degenerate ranges reported by the device', () {
-    test('a range with no headroom on one side clamps that side to 0', () {
-      // A device that reports max == 0 can only cut. Asking for a boost must
-      // resolve to "no change", never to a negative value.
+    test('a device with no headroom above unity can never boost', () {
       expect(mapearGananciaNativa(12, minDecibels: -15, maxDecibels: 0), 0);
-      expect(mapearGananciaNativa(-12, minDecibels: -15, maxDecibels: 0), -15);
+      expect(mapearGananciaNativa(6, minDecibels: -15, maxDecibels: 0), 0);
+    });
+
+    test('a cut the device could honour exactly is not over-delivered', () {
+      // CONTRACT CHANGE: this used to answer -15, spending the device's whole
+      // range on a request for -12 dB. The user asked for -12; -12 is
+      // representable here, so -12 is what is sent.
+      expect(mapearGananciaNativa(-12, minDecibels: -15, maxDecibels: 0), -12);
     });
 
     test('a zero-width range collapses everything to 0', () {