fix(visualizador): add a discrete-bar rendering mode

Audit 1.7/2.5 (t4 lines 66-68, 120-122): the prototype draws 30
discrete bottom-anchored bars (radius 2, vertical gradient); the
build's VisualizadorAudio painted a single continuous oscilloscope
stroke, so its `barras: 30` parameter never produced bars.

Adds an opt-in `barrasDiscretas` mode (default false, byte-identical
continuous rendering preserved for any other caller) plus a
`gradienteFinAlpha` knob for the two screens' differing gradient
end-alpha (.3 vs .45). Wires it into the Escuchar hero and the full
player, correcting the player's bar count/height to match the
prototype (26->30 bars, 46->40px) at the same time.

lib/pantallas/pantalla_inicio.dart

@@ -291,6 +291,9 @@ class _EscucharHero extends StatelessWidget {
                         barras: 30,
                         altura: 26,
                         color: context.pluriTokens.liveGreen,
+                        // Audit 1.7 (t4 lines 66-68): 30 discrete
+                        // bottom-anchored bars, not a continuous stroke.
+                        barrasDiscretas: true,
                       ),
                     ],
                   ),

lib/pantallas/pantalla_reproductor.dart

@@ -152,6 +152,7 @@ class _PantallaReproductorState extends State<PantallaReproductor> {
               ).pluriFadeIn(context, delay: const Duration(milliseconds: 230)),
               const SizedBox(height: 14),
               PluriGlassSurface(
+                key: const Key('player-visualizer'),
                 borderRadius: BorderRadius.circular(tokens.radiusLg),
                 padding: const EdgeInsets.symmetric(
                   horizontal: 14,
@@ -161,9 +162,13 @@ class _PantallaReproductorState extends State<PantallaReproductor> {
                   estadoStream: estado.estadoStream,
                   androidAudioSessionIdStream:
                       estado.audio.androidAudioSessionIdStream,
-                  barras: 26,
+                  // Audit 2.5 (t4 lines 120-122): 30 discrete bars at 40px,
+                  // gradient ending at 45% alpha (not the hero's 30%).
+                  barras: 30,
                   color: tokens.warmCoral,
-                  altura: 46,
+                  altura: 40,
+                  barrasDiscretas: true,
+                  gradienteFinAlpha: 0.45,
                 ),
               ).pluriFadeIn(context, delay: const Duration(milliseconds: 270)),
               const SizedBox(height: 22),