fix(cd): paused/stopped/seeking drive delivers I2S silence, holds position

The SSI head streamed disc data unconditionally: through Pause/Stop,
during seeks, and past track ends into data sectors.  A game with its
DSP CD mix enabled plays those data sectors as full-scale noise --
device-reported as "loud clipped static" under music and SFX (Primal
Rage, Baldies, Iron Soldier 2 garbage-audio loop).

A real drive outputs silence and holds position while paused, stopped,
or seeking; the I2S bit clock keeps running.  Gate the sample fetch on
cdPlaying / seekDelay: deliver zeros without advancing the head, but
keep the DSP SSI interrupt firing (the synth engine is clocked off it
in slave mode).

Verified: make test green, HLE sweep 6/9 and BIOS sweep 8/9 at
baseline, Primal Rage attract music timeline unchanged.

Co-Authored-By: Claude Fable 5 <[email protected]>

src/cd/cdrom.c

@@ -1706,6 +1706,20 @@ void SetSSIWordsXmittedFromButch(void)
    if (ssiXmitCount <= 5 || (ssiXmitCount % 10000) == 0)
       CD_LOG("SSI xmit #%u: ssiBufPtr=%u ssiBlock=%u cdPlaying=%d\n",
              ssiXmitCount, ssiBufPtr, ssiBlock, cdPlaying);
+
+   /* A paused/stopped/seeking drive outputs silence and holds position;
+    * the I2S bit clock keeps running (the DSP synth is clocked off these
+    * interrupts in slave mode) but no disc data moves.  Without this
+    * gate the head streamed forever -- through pauses and past track
+    * ends into data sectors, which a game with its CD mix enabled plays
+    * as full-scale noise ("loud clipped static"). */
+   if (!cdPlaying || seekDelay > 0)
+   {
+      lrxd = 0;
+      rrxd = 0;
+      return;
+   }
+
    // Advance by 4 bytes (one stereo sample: 2 bytes L + 2 bytes R).
    // Uses the SSI head's own cursor -- see ssiBuf declaration for why
    // this must not share cdBufPtr with the FIFO read path.