cdrom: cancel stale INT2 on new command (PSX-CD-007)

Cancel the previous command's outstanding second-response interrupt when
a new CD command is accepted (psx-spx command flow; DuckStation
BeginCommand semantics). Without this, a Pause(0x09) INT2 survives into
the guest's next Setmode/Setloc/ReadN sequence and fires mid-read as a
stale COMPLETE — LIBDS drivers treat it as a read failure, corrupting
the streamed resource chain.

Cross-validated: C-12 Final Resistance mission 1 PC=0 null-callback
termination disappeared after absorbing this fix (DEVLOG 2026-08-05).
MOHU Mission 1 freeze in a DsRead FATAL/retry loop on single-sector
marker reads.

Also guard beetle SIGSTOP trap behind #ifndef _WIN32 (Windows has no
SIGSTOP).

(cherry picked from commit 2c43fc69c66242a86183156a6f1e60b2f81b1e47)
(cherry picked from commit c03fe5ba3d49241dfaedb344db6ad9c977070722)

runtime/src/beetle_libretro.cpp

@@ -75,7 +75,9 @@ static void cdcmd_trace_callback(uint8_t cmd, uint8_t nargs,
                                  uint32_t pc) {
     /* Symmetric self-stop trap (mirror of the runtime's PSX_CD_TRAP_CMD):
      * SIGSTOP on the Nth matching CD command so a debugger can freeze the
-     * oracle at the exact boot point and diff its state against ours. */
+     * oracle at the exact boot point and diff its state against ours.
+     * POSIX-only, same guard as debug_server.c — Windows has no SIGSTOP. */
+#ifndef _WIN32
     {
         static long trap_cmd = -2, trap_nth = 1, hits = 0;
         if (trap_cmd == -2) {
@@ -86,6 +88,7 @@ static void cdcmd_trace_callback(uint8_t cmd, uint8_t nargs,
         }
         if ((long)cmd == trap_cmd && ++hits == trap_nth) raise(SIGSTOP);
     }
+#endif
     BeetleCdcmdEntry *e = &s_cdcmd_trace[s_cdcmd_idx];
     e->seq = s_cdcmd_seq++; e->pc = pc; e->cmd = cmd; e->nargs = nargs;
     e->a0 = a0; e->a1 = a1; e->a2 = a2;