ci(ios-metal): enable Metal API validation in screenshot suite

[shai-almog]

Summary

Wire Apple's Metal API Validation into the existing build-ios-metal screenshot job so the class of bug that produced #5103 (a render pass binding a pipeline whose stencilAttachmentPixelFormat does not match the pass's stencil attachment) trips CI the moment it is committed, rather than four releases later in a user-built iOS app.

The plumbing is intentionally narrow:

• scripts/run-ios-ui-tests.sh reads MTL_DEBUG_LAYER and MTL_DEBUG_LAYER_ERROR_MODE from the caller's environment and forwards them to the launched simulator app via simctl --setenv. When neither is set the launch line is byte-identical to before.
• .github/workflows/scripts-ios.yml sets both vars (assert mode) on the Metal job's test step. The GL job is untouched because Metal validation only fires under the Metal layer.

The shape on the Metal job: any validation error crashes the app, the suite stops emitting CN1SS:SUITE:FINISHED, the test step times out, the job goes red. Failure surface is loud; the simctl-launch.log and the existing log show --last 30m fallback both capture the assertion message for diagnosis.

Why not just rely on the merged #5103 fix

Same bug, different vector. The #5103 fix patched one seed-pass site. This change is the structural guard that would have caught it (and will catch the next one) regardless of which file the mistake lives in.

Static audit done before flipping

Before enabling I scanned every MTLRenderPassDescriptor creation and every setRenderPipelineState: call in CN1Metalcompat.m, CN1MetalPipelineCache.m, and METALView.m:

Site	Binds a pipeline?	Stencil attached?
CN1Metalcompat.m:1407 clearPass	No (clear + endEncoding)	n/a
CN1Metalcompat.m:1430 seedPass (post-#5103)	Yes	Yes (1449)
CN1Metalcompat.m:1522 BeginMutableImageDraw	Yes (via activeEncoder)	Yes (1544)
METALView.m:323 clearPass	No (clear + endEncoding)	n/a
METALView.m:354 screen pass	Yes (via activeEncoder)	Yes (371)

The pre-#5103 framebufferOnly validation assertion noted at METALView.m:145 was already fixed by metalLayer.framebufferOnly = NO at line 149. So the first CI run on this branch should pass cleanly. If it doesn't, validation has surfaced a sibling latent bug, which is exactly the point — we'd address it in a follow-up or revert this one env-var block until that's done.

Test plan

• First Metal CI run on this branch completes the screenshot suite without MTL_DEBUG_LAYER_ERROR_MODE=assert triggering. If it does trigger, dig into simctl-launch.log + the fallback log show capture in the uploaded ios-ui-tests-metal artifact.
• GL job is untouched and stays green.
• Local sanity: bash -n scripts/run-ios-ui-tests.sh clean; bash-3.2 array-expansion idiom verified empty + populated under set -u.

🤖 Generated with Claude Code

[shai-almog]

Compared 122 screenshots: 122 matched.
✅ Native iOS screenshot tests passed.

Benchmark Results

• VM Translation Time: 0 seconds
• Compilation Time: 262 seconds

Build and Run Timing

Metric	Duration
Simulator Boot	67000 ms
Simulator Boot (Run)	1000 ms
App Install	12000 ms
App Launch	40000 ms
Test Execution	344000 ms

Detailed Performance Metrics

Metric	Duration
Base64 payload size	8192 bytes
Base64 benchmark iterations	6000
Base64 native encode	944.000 ms
Base64 CN1 encode	1827.000 ms
Base64 encode ratio (CN1/native)	1.935x (93.5% slower)
Base64 native decode	417.000 ms
Base64 CN1 decode	1063.000 ms
Base64 decode ratio (CN1/native)	2.549x (154.9% slower)
Base64 SIMD encode	388.000 ms
Base64 encode ratio (SIMD/native)	0.411x (58.9% faster)
Base64 encode ratio (SIMD/CN1)	0.212x (78.8% faster)
Base64 SIMD decode	387.000 ms
Base64 decode ratio (SIMD/native)	0.928x (7.2% faster)
Base64 decode ratio (SIMD/CN1)	0.364x (63.6% faster)
Image encode benchmark iterations	100
Image createMask (SIMD off)	75.000 ms
Image createMask (SIMD on)	11.000 ms
Image createMask ratio (SIMD on/off)	0.147x (85.3% faster)
Image applyMask (SIMD off)	168.000 ms
Image applyMask (SIMD on)	105.000 ms
Image applyMask ratio (SIMD on/off)	0.625x (37.5% faster)
Image modifyAlpha (SIMD off)	166.000 ms
Image modifyAlpha (SIMD on)	122.000 ms
Image modifyAlpha ratio (SIMD on/off)	0.735x (26.5% faster)
Image modifyAlpha removeColor (SIMD off)	224.000 ms
Image modifyAlpha removeColor (SIMD on)	91.000 ms
Image modifyAlpha removeColor ratio (SIMD on/off)	0.406x (59.4% faster)
Image PNG encode (SIMD off)	1266.000 ms
Image PNG encode (SIMD on)	1003.000 ms
Image PNG encode ratio (SIMD on/off)	0.792x (20.8% faster)
Image JPEG encode	673.000 ms

[shai-almog]

Compared 109 screenshots: 109 matched.
✅ Native iOS Metal screenshot tests passed.

Benchmark Results

• VM Translation Time: 0 seconds
• Compilation Time: 377 seconds

Build and Run Timing

Metric	Duration
Simulator Boot	79000 ms
Simulator Boot (Run)	1000 ms
App Install	16000 ms
App Launch	13000 ms
Test Execution	1505000 ms