refactor(core/audio): helper fadeProgress compartit per fades (DUP-03)

updateOutgoingFade i updateIncomingFade calculaven el mateix PERCENT
lineal (ELAPSED / duration_ms) per separat i prenien decisions sobre si
el fade havia acabat comparant bytes. Ara fadeProgress(FadeState)
retorna [0..1] i concentra la corba del fade — si cal passar-la a
logarítmica o quadràtica, es toca en un sol lloc.

També protegeix duration_ms<=0 com a "fade completat" (abans el divisor
donava una divisió per zero en debug o un progrés >> 1 en release).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/audit-core-audio.md

@@ -32,7 +32,7 @@ Los IDs se mantienen estables una vez asignados. Cada entrada apunta al(los) fic
 |----|---------------|----------|-----------|--------|-------|
 | DUP-01 | `audio.cpp:58-101` | Dos `playMusic` (nombre/puntero) repiten el bloque crossfade/stop+play | Menor | DONE | `playMusicInternal(Ja::Music*, loop, crossfade_ms)` privat; els dos overloads fan gating + name-handling i deleguen el dispatch |
 | DUP-02 | `audio.cpp:172-185` | `setSoundVolume`/`setMusicVolume` idéntica estructura de clamp+gate+producto | Menor | DONE | `effectiveVolume(volume, channel_enabled)` privat; els dos setters deleguen |
-| DUP-03 | `jail_audio.hpp:196-222` | `updateOutgoingFade`/`updateIncomingFade` comparten la curva lineal | Menor | PENDING | Helper `fadeProgress(FadeState&) -> float` |
+| DUP-03 | `jail_audio.hpp:196-222` | `updateOutgoingFade`/`updateIncomingFade` comparten la curva lineal | Menor | DONE | `fadeProgress(FadeState&)` concentra la corba en un sol lloc; també cobreix `duration_ms <= 0` com a 1.0 |
 | DUP-04 | `jail_audio.hpp:391-471` | `fadeOutMusic`/`crossfadeMusic` repiten "destruir outgoing + pre-fill + robar stream + reset vorbis" | Importante | PENDING | Función privada `stealCurrentIntoOutgoing(int ms)` |
 | DUP-05 | `jail_audio.hpp:575-631` | `pauseChannel`/`resumeChannel`/`stopChannel` repiten dispatch `channel==-1 / [0,MAX)` | Menor | PENDING | Helper `forEachTargetChannel(int, fn)` |
 

source/core/audio/jail_audio.hpp

@@ -197,18 +197,27 @@ namespace Ja {
 
     // --- Core Functions ---
 
+    // Retorna el progrés lineal [0..1] d'un fade. 1.0 vol dir completat. Única
+    // font de la corba del fade: si es vol canviar a logarítmica/quadràtica,
+    // s'edita aquí i afecta fade-in i fade-out alhora.
+    inline auto fadeProgress(const FadeState& fade) -> float {
+        if (fade.duration_ms <= 0) { return 1.0F; }
+        const Uint64 ELAPSED = SDL_GetTicks() - fade.start_time;
+        if (ELAPSED >= static_cast<Uint64>(fade.duration_ms)) { return 1.0F; }
+        return static_cast<float>(ELAPSED) / static_cast<float>(fade.duration_ms);
+    }
+
     // Avança el fade-out de l'outgoing music (crossfade o fade-out a silenci).
     inline void updateOutgoingFade() {
         if (outgoing_music.stream == nullptr || !outgoing_music.fade.active) { return; }
 
-        const Uint64 ELAPSED = SDL_GetTicks() - outgoing_music.fade.start_time;
-        if (ELAPSED >= static_cast<Uint64>(outgoing_music.fade.duration_ms)) {
+        const float PROGRESS = fadeProgress(outgoing_music.fade);
+        if (PROGRESS >= 1.0F) {
             SDL_DestroyAudioStream(outgoing_music.stream);
             outgoing_music.stream = nullptr;
             outgoing_music.fade.active = false;
         } else {
-            const float PERCENT = static_cast<float>(ELAPSED) / static_cast<float>(outgoing_music.fade.duration_ms);
-            SDL_SetAudioStreamGain(outgoing_music.stream, outgoing_music.fade.initial_volume * (1.0F - PERCENT));
+            SDL_SetAudioStreamGain(outgoing_music.stream, outgoing_music.fade.initial_volume * (1.0F - PROGRESS));
         }
     }
 
@@ -216,13 +225,12 @@ namespace Ja {
     inline void updateIncomingFade() {
         if (!incoming_fade.active) { return; }
 
-        const Uint64 ELAPSED = SDL_GetTicks() - incoming_fade.start_time;
-        if (ELAPSED >= static_cast<Uint64>(incoming_fade.duration_ms)) {
+        const float PROGRESS = fadeProgress(incoming_fade);
+        if (PROGRESS >= 1.0F) {
             incoming_fade.active = false;
             SDL_SetAudioStreamGain(current_music->stream, music_volume);
         } else {
-            const float PERCENT = static_cast<float>(ELAPSED) / static_cast<float>(incoming_fade.duration_ms);
-            SDL_SetAudioStreamGain(current_music->stream, music_volume * PERCENT);
+            SDL_SetAudioStreamGain(current_music->stream, music_volume * PROGRESS);
         }
     }