Fix PinMAME busy-wait freeze on browser/window focus loss

core.vbs auto-wires Controller.Pause = True into every table's Paused
event, which Player::OnFocusChanged() fires on focus loss. PinMAME's
own pause handling in usrintrf.c is a "while (g_fPause) { ...;
draw_screen(); ... }" busy-wait written for a real-OS-thread host,
where a separate thread later flips g_fPause back to 0. Under our
single-threaded Emscripten cooperative-scheduling model there is no
second thread to clear it, so entering that loop spins forever at
100% CPU with the tab completely unresponsive.

cpu_run_emscripten_step() now checks g_fPause up front and skips
cpu_timeslice() (and therefore that loop) entirely while paused;
emulation simply doesn't advance until Controller.Pause is set back
to False and the step function is called again next frame.

Confirmed fixed by hands-on testing: unfocusing the browser during a
PinMAME ROM session no longer freezes the tab.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

patches/pinmame/0004-emscripten-cooperative-scheduling.patch

@@ -1,8 +1,8 @@
 diff --git a/src/cpuexec.c b/src/cpuexec.c
-index 07db6a1..0bfcbe7 100644
+index 07db6a1..19da31a 100644
 --- a/src/cpuexec.c
 +++ b/src/cpuexec.c
-@@ -466,6 +466,65 @@ void cpu_run(void)
+@@ -466,6 +466,78 @@ void cpu_run(void)
  #endif
  }
  
@@ -29,6 +29,19 @@ index 07db6a1..0bfcbe7 100644
 +   once it has fully quit (cpu_post_run() has already been invoked). */
 +int cpu_run_emscripten_step(void)
 +{
++	/* usrintrf.c's per-frame video update contains a "while (g_fPause) {
++	   ...; draw_screen(); ... }" busy-wait (see the VPINMAME/LIBPINMAME
++	   branch around updatescreen()) that assumes a real OS thread will
++	   later flip g_fPause back to 0 from outside it. We're single-threaded
++	   here, so entering that loop at all would spin forever burning 100%
++	   CPU with no way out - nothing else can ever run to clear the flag.
++	   Skip cpu_timeslice() (and therefore that loop) entirely while paused;
++	   the emulation simply doesn't advance until Controller.Pause is set
++	   back to False and this function is called again next frame. */
++	extern int g_fPause;
++	if (g_fPause)
++		return 1;
++
 +	const double frame_target = timer_get_time() + (1.0 / 60.0);
 +
 +	while (!time_to_quit && !time_to_reset && timer_get_time() < frame_target)