feat(paises): add country browser and extract shared radio transport

Extracts ServicioRadio's transport loop (server discovery, host rotation,
bounded retries, User-Agent, timeout, status check, json.decode, sticky-host
bookkeeping) out of `_get` into a new `_getJson(path, params) ->
Future<List<dynamic>>` helper, moved as one block with no logic edits. `_get`
is reimplemented on top, still owning every station-specific concern:
`lastcheckok: '1'`, `Emisora.fromApi` + the empty-uuid/url filter, and the
`_compararCalidad` quality sort. `_getJson` is deliberately sort-agnostic and
filter-agnostic so a non-station endpoint can reuse the resilience behaviour
without inheriting station-only semantics.

Non-negotiable ordering followed per design ADR-4: new
test/servicios/servicio_radio_transporte_test.dart characterises all 8
existing station calls (7 via `_get` plus `registrarClick`, which builds its
own URI) against the UNMODIFIED `_get` first - green by construction -
pinning path, lastcheckok=1, hidebroken=true, a non-empty User-Agent, exact
order/reverse/limit/offset, and the exact returned UUID sequence from a
fixture with deliberately shuffled bitrate/clickcount/votes. That last
assertion is what makes the extraction safe: a sort that silently sank into
transport would pass every other check. Re-running the same file after the
extraction is byte-identical green. test/servicios/servicio_radio_test.dart
is untouched by this work unit - its passing unmodified is itself a signal
that transport wasn't disturbed.

The 6 pre-existing `order: bitrate` occurrences (obtenerPopulares,
buscarPorNombre, buscarPorPais, buscarPorIdioma, buscarPorTag, buscar) are
untouched - a deliberate server-side quality bias deciding which stations
return within `limit`, unrelated to and never to be confused with the
user-facing "Ordenar" control, which stays entirely client-side via the
existing OrdenEmisoras (Engram reference/radio-browser-sort-order).

Behaviour delta, accepted per ADR-4, not a regression: moving
`_servidorActual` bookkeeping into `_getJson` means a successful
`/json/countries` call now warms the sticky host for subsequent station
calls too - one shared warm mirror per instance, desirable, not per-call-type
state.

Adds the Paises browser over the verified `/json/countries` contract (Engram
reference/radio-browser-countries-endpoint): new lib/modelos/pais_radio.dart
(`PaisRadio.fromApi` parses `stationcount` via `int.tryParse` since the API
returns it as a JSON string, not an int - an `as int` cast would throw),
`obtenerPaises()` sends neither `lastcheckok` nor `order` (the screen sorts
client-side by name; the API's raw byte order isn't proper collation for any
locale this app ships), and inherits `hidebroken=true` from the unchanged
`_uri` (desirable here too, since the endpoint's own default is false).
`EstadoBusqueda` gains `paises`/`cargandoPaises`/`cargarPaises()` with an
in-memory cache guard so re-entering the screen never refetches.

New PantallaPaises (lib/pantallas/pantalla_paises.dart): a "Tus idiomas"
shortlist (one representative country per the app's 13 supported locales,
matched against the fetched list - the proposal/spec name this section but
don't specify its derivation) above the full alphabetical list, each entry
showing its parsed station count. Reachable from Buscar's discovery landing
state via a new entry row, added now rather than left dangling per this
file's own forward-reference comment (and the WU15/WU15b lesson: a
fully-tested but unreachable screen is a real defect, not a follow-up).

New ARB keys (en/es only, matching this change's established precedent):
countriesScreenTitle, countriesYourLanguagesTitle, countriesAllTitle,
radioCountriesError.

Tests: 631 -> 649 (2 skipped, unchanged). flutter analyze unchanged at 1
pre-existing info. grep confirms `countrycodes` appears nowhere in lib/.

openspec/changes/rediseno-funcional/tasks.md

@@ -467,28 +467,28 @@ Calls Unchanged by Transport Extraction
 > **Special ordering rule (non-negotiable): task 7.1 must be green before task 7.4 begins.** Its shuffled-fixture
 > UUID-sequence assertion is what makes the extraction safe — skipping it lets a sort silently sink into transport.
 
-- [ ] 7.1 RED→GREEN (characterisation, green by construction against the **unmodified** `_get`) — create
+- [x] 7.1 RED→GREEN (characterisation, green by construction against the **unmodified** `_get`) — create
       `servicio_radio_transporte_test.dart` pinning, for each of the 7 `_get`-based methods (`obtenerPopulares`,
       `obtenerTendencias`, `buscarPorNombre`, `buscarPorPais`, `buscarPorIdioma`, `buscarPorTag`, `buscar`): request
       path, `lastcheckok=1` present, `hidebroken=true` present, non-empty `User-Agent`, exact
       `order`/`reverse`/`limit`/`offset`, and the exact returned UUID sequence from a fixture with deliberately
       shuffled `bitrate`/`clickcount`/`votes`. Pin `registrarClick`'s path and that it sends *some* `User-Agent`
       (do **not** pin its known-stale literal at line 325 — that would convert a bug into a contract).
-- [ ] 7.2 RED — `PaisRadio.fromApi` parses `stationcount` via `int.tryParse` from a fixture where it arrives as a
+- [x] 7.2 RED — `PaisRadio.fromApi` parses `stationcount` via `int.tryParse` from a fixture where it arrives as a
       JSON **string**; defaults safely on missing fields.
-- [ ] 7.3 RED — `estado_busqueda_test.dart` additions for `paises`, `cargandoPaises`, `cargarPaises()` (in-memory
+- [x] 7.3 RED — `estado_busqueda_test.dart` additions for `paises`, `cargandoPaises`, `cargarPaises()` (in-memory
       cache guard); `pantalla_paises_test.dart` for "Tus idiomas" + full alphabetical list with counts.
-- [ ] 7.4 GREEN — extract `_getJson` (transport only: `_descubrirServidores`, host rotation, bounded retries,
+- [x] 7.4 GREEN — extract `_getJson` (transport only: `_descubrirServidores`, host rotation, bounded retries,
       User-Agent, timeout, status check, `json.decode`, sticky-host bookkeeping) out of `_get`, moved as **one
       block, no logic edits**; `_get` re-implemented on top, still applying `lastcheckok`, `Emisora.fromApi`,
       `_compararCalidad`.
-- [ ] 7.5 GREEN — implement `lib/modelos/pais_radio.dart`, `obtenerPaises()` (calls
+- [x] 7.5 GREEN — implement `lib/modelos/pais_radio.dart`, `obtenerPaises()` (calls
       `_getJson('/json/countries', const {})`, no `lastcheckok`, no `order`), and `lib/pantallas/pantalla_paises.dart`.
-- [ ] 7.6 RE-RUN (proof step) — re-run task 7.1's tests against the extracted code; must be byte-identical green,
+- [x] 7.6 RE-RUN (proof step) — re-run task 7.1's tests against the extracted code; must be byte-identical green,
       including the shuffled-fixture UUID-sequence assertion.
-- [ ] 7.7 REFACTOR — document in the commit body that a successful `/json/countries` call now warms
+- [x] 7.7 REFACTOR — document in the commit body that a successful `/json/countries` call now warms
       `_servidorActual` for subsequent station calls — an accepted, intentional behaviour delta, not a regression.
-- [ ] 7.8 Verify — grep confirms `countrycodes` appears nowhere in `lib/`; the string-`stationcount` fixture renders
+- [x] 7.8 Verify — grep confirms `countrycodes` appears nowhere in `lib/`; the string-`stationcount` fixture renders
       `482` without a cast error; the countries request has no `lastcheckok`; all 8 original methods' tests pass
       identically before and after.