fix: wire cdrom_tick into vblank interval, fire IRQ on command completion

Two CDROM issues preventing BIOS from progressing past Sony logo:

1. cdrom_tick() was never called — delayed CDROM responses (GetStatus,
   Init, Pause) never completed because pending.delay was never
   decremented. Wired into psx_check_interrupts vblank interval.

2. exec_command() called set_irq() but not fire_cdrom_irq() for
   immediate responses (GetStat, SetLoc, Demute, etc.). The CDROM
   IRQ flag was set internally but i_stat bit 2 was never raised,
   so the BIOS exception handler never delivered CDROM events.

BIOS still stalls at same draw count — next bug in chain is EvCB
event delivery (TestEvent returns 0 because VBlank events aren't
being written to the event control block table).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

runtime/src/cdrom.c

@@ -286,6 +286,10 @@ static void exec_command(uint8_t cmd) {
     }
 
     param_count = 0;
+
+    /* Fire the CDROM IRQ for the immediate response.
+     * Delayed responses (pending) fire from process_pending(). */
+    fire_cdrom_irq();
 }
 
 static void process_pending(void) {

runtime/src/interrupts.c

@@ -29,6 +29,7 @@
 #include "sio.h"
 #include "timers.h"
 #include "gpu.h"
+#include "cdrom.h"
 #include "cpu_state.h"
 #include <stdio.h>
 #include <stdlib.h>
@@ -102,6 +103,7 @@ void psx_check_interrupts(CPUState* cpu) {
             i_stat |= (1 << IRQ_VBLANK);
             gpu_vblank_tick();  /* Toggle LCF (GPUSTAT bit 31) */
             timers_tick(33868); /* ~1 NTSC frame worth of cycles */
+            cdrom_tick();      /* Process pending CDROM responses */
         }
     }