fix(tests): make the platform-specific suites portable and deterministic

Four independent CI failures the first real cross-platform run surfaced:

- macOS SIGSEGV: the real-GL unit tests in gl_bridge ran wherever CGL is
  available, including the headless CI runner. Gate them with the same
  OAK_GPU_TESTS switch the integration GL tests already use (skip on CI,
  opt in on a real Mac).
- Windows build: examples compile under `cargo test`, and bench_playback
  used libc::getrusage unconditionally. Keep the Unix CPU accounting
  behind #[cfg(unix)] and report zero CPU seconds elsewhere.
- openKylin arm64: engine_without_a_project_hits_the_guard_paths assumed
  the library backend was unconfigured while a parallel config test
  transiently set Storage/Backend=sqlite. Take the shared config lock and
  pin the key off for the test's duration.
- openKylin x64: the prefetch smoke test asserted an exact decode count,
  but the hand-off LRU holds only DECODE_LRU_CAP (2) frames, so a request
  can miss the prefetched copy under scheduling pressure and re-run the
  producer (the eval cache still serves the pixels). Bound the count
  instead of pinning it; the deterministic sibling test pins read-ahead
  usage.

crates/oak-render/tests/render_threads_test.rs

@@ -913,10 +913,25 @@ fn pipeline_playback_prefetches_ahead_of_the_render() {
 		times.len() as u64,
 		"every playback post queued its footage prefetch"
 	);
-	assert_eq!(
+	// Each distinct frame is produced at least once. The hand-off LRU holds
+	// only `DECODE_LRU_CAP` (2) frames, so under scheduling pressure a
+	// request can miss the prefetched copy and re-run the producer; the
+	// eval-side cache still serves the pixels without touching the media,
+	// but the counter moves. Bound this to one production per command
+	// (prefetch + request) instead of asserting an exact count; the
+	// deterministic `pipeline_prefetch_is_the_frame_the_render_request_uses`
+	// pins that the read-ahead is actually used.
+	assert!(
+		stats.decodes >= times.len() as u64,
+		"each distinct frame is produced at least once ({} < {})",
 		stats.decodes,
-		times.len() as u64,
-		"each distinct frame decodes exactly once (prefetch or rendezvous)"
+		times.len()
+	);
+	assert!(
+		stats.decodes <= 2 * times.len() as u64,
+		"at most one production per prefetch and per request ({} > {})",
+		stats.decodes,
+		2 * times.len()
 	);
 	// Note: `procpool::main_heap_frame_copies` only counts the shm path,
 	// which the thread pipeline never touches, so asserting it here would