fix: replace seam resets with exact virtual loop feeding for stretch engines

Feed Signalsmith and Bungee a mathematically continuous virtual loop
source instead of padding/reseeking at boundaries. Adds wrapLoopPosition,
reflectPingPongPosition, readExactLoopSample, and advanceStretchSrcPos
helpers. Removes stretchRestartPending, Bungee ping-pong crossfade
fields/helpers, and all seam-reset logic from loop/ping-pong playback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: tucktuckg00se

src/audio/Voice.h

@@ -67,10 +67,4 @@ struct Voice
     double       bungeePitch        = 1.0;
     double       bungeeSpeed        = 1.0;
     bool         bungeeResetNeeded  = false;
-
-    // Bungee ping-pong crossfade fields
-    int          bungeePPFade       = 0;   // remaining fade samples
-    int          bungeePPFadeLen    = 64;  // configured fade buffer size, updated from sample rate
-    int          bungeePPFadeActiveLen = 0;
-    std::vector<float> bungeePPFadeL, bungeePPFadeR;
 };