metal: pause step 3b pending sx-side fixes (filed 0024-0030)

Step 3b code is wired across UIRenderer + GlyphCache + UIPipeline +
chess game (gpu_mode = .metal on iOS, MetalGPU bound via the GPU
protocol). macOS GL chess, iOS-sim GLES chess, and iOS-sim Metal
triangle (63-metal-clear.sx) all still render.

iOS-sim Metal chess crashes inside replaceRegion uploading the 1MB
font atlas. Bisecting that crash exposed several sx-language issues
where mid-bisect tracers (NSLog inside if/else branch bodies) didn't
produce output, blocking further investigation.

Filing each finding as examples/issue-NNNN.sx rather than working
around piecemeal:

Bugs:
- 0024 NSLog/foreign-call inside if/else body not producing output
- 0025 C-ABI param coercion incomplete for composites >16B
       (combined direct-call abiCoerceParamType TODO + call_indirect
        path that doesn't apply C-ABI coercion at all)
- 0026 replaceRegion 1MB upload crash (likely downstream of 0025)

Features needed for step 4 + cleanup:
- 0027 Obj-C block bridge (^{...}) for animateWithDuration:
- 0028 Optional protocol box (?GPU = null) replaces T = ---; has_T: bool
- 0029 destroy_texture/buffer/shader on GPU protocol
- 0030 extern cross-file globals

Library-side: renderer.sx + glyph_cache.sx + pipeline.sx gain a
`gpu: GPU = ---; has_gpu: bool` field pair + branches that route every
GL touchpoint through the protocol when has_gpu. glyph_cache.init
saves/restores those fields around its memset. pipeline.set_gpu()
propagates to renderer + font. Renderer's MSL shader source added as
UI_MSL_SRC using packed_float2/packed_float4 to keep the 12-float
interleaved vertex layout tight (48 bytes).

metal.sx: dual-phase init (init(null, 0, 0) for eager device+queue,
re-init with the layer once UIKit installs the SxMetalView).
setStorageMode:.shared on every texture descriptor to ensure CPU-
writable atlas pixels on Apple Silicon iOS-sim.

Regression suite: 68 passing, 0 failed. WASM chess build currently
broken under step 3b state (silent compiler crash); documented in
CHECKPOINT.md, likely fallout from one of the filed issues (probably
0028 — the verbose protocol-box pattern). Step 3b resumes after
0024-0030 land.

examples/issue-0026.sx

@@ -0,0 +1,68 @@
+// issue-0026: Chess game on iOS-sim with `plat.gpu_mode = .metal` crashes
+// inside `[MTLTexture replaceRegion:mipmapLevel:withBytes:bytesPerRow:]`
+// when uploading the 1024×1024 R8 font atlas. The 1×1 RGBA8 white tex
+// through the SAME code path (metal_update_texture_region_ios in
+// library/modules/gpu/metal.sx) works.
+//
+// Blocked on issue-0024 (NSLog inside if/else not firing — or unified-log
+// buffer loss on crash; investigation pending) — without a trustworthy
+// tracer we can't reliably bisect which arg arrives wrong. Most likely
+// cause: this is downstream of issue-0025's ABI gaps (MTLRegion is 48
+// bytes and goes through `xx objc_msgSend` cast, which is the
+// call_indirect path that issue-0025 part B covers).
+//
+// ── Reproduction recipe ───────────────────────────────────────────────────
+//
+//   cd /Users/agra/projects/game
+//   /Users/agra/projects/sx/zig-out/bin/sx build --target ios-sim main.sx \
+//     --bundle sx-out/ios/SxChess.app --bundle-id co.swipelab.sxchess \
+//     -F ~/Library/Frameworks
+//   cp -R assets sx-out/ios/SxChess.app/
+//   codesign --force --sign - --timestamp=none sx-out/ios/SxChess.app
+//   xcrun simctl install booted sx-out/ios/SxChess.app
+//   xcrun simctl launch --terminate-running-process booted co.swipelab.sxchess
+//   sleep 4 && xcrun simctl io booted screenshot /tmp/sx-chess.png
+//
+// Expected (after fix): chess board renders via Metal.
+// Observed: app launches, returns immediately to home screen, no screen
+// touched. The simpler examples/63-metal-clear.sx demo still renders the
+// colored triangle on the same sim, so the Metal pipeline itself works
+// for small uploads.
+//
+// ── Candidate root causes (in priority order) ─────────────────────────────
+//
+// 1. issue-0025 fallout (most likely): MTLRegion (48 B by value) passed
+//    via the *MTLRegion workaround. The call_indirect path (issue-0025
+//    part B) doesn't ABI-coerce, so the pointer-shaped declaration may
+//    not actually pass the address in the right register slot for that
+//    call site shape (6 args, including the indirect aggregate).
+//
+// 2. iOS-sim Metal-driver limitation: `setStorageMode:.shared` may not be
+//    honored for r8 textures of this size; default may be `.private`
+//    which precludes CPU-side replaceRegion. Workaround would be to
+//    upload via `MTLBuffer` + `MTLBlitCommandEncoder` (newBufferWithBytes
+//    + copyFromBuffer:sourceOffset:sourceBytesPerRow:...:toTexture:...).
+//
+// 3. sx-side `xx` cast bug: bytes_per_row : u64 = xx (u32_expr) may
+//    truncate or sign-extend incorrectly. Less likely (the math comes
+//    out to 1024, which fits in any width).
+//
+// ── How to resolve ────────────────────────────────────────────────────────
+//
+// After issues 0024 + 0025 are landed:
+//   1. Re-add the trace NSLog markers ("[metal] U1..U5" in
+//      metal_update_texture_region_ios) — now they should actually print.
+//   2. Re-build + relaunch chess on iOS-sim.
+//   3. If U5 fires after U4 (no crash inside msg_replace), the bug was
+//      ABI-related; declare success and rename this file to
+//      examples/NN-metal-large-region-upload.sx (next free NN).
+//   4. If U4 → crash persists, fall back to the MTLBuffer + blit
+//      encoder path in metal.sx's create_texture (when pixels != null,
+//      allocate a temporary MTLBuffer with newBufferWithBytes:length:options:
+//      then run a one-shot command buffer with a MTLBlitCommandEncoder
+//      copying the buffer into the texture). This is the Apple-recommended
+//      approach for large texture initial-uploads.
+
+#import "modules/std.sx";
+
+main :: () -> s32 { 0; }