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.

library/modules/gpu/metal.sx

@@ -28,6 +28,12 @@ MTL_PIXEL_FORMAT_R8_UNORM    :u64: 10;
 MTL_LOAD_ACTION_CLEAR  :u64: 2;
 MTL_STORE_ACTION_STORE :u64: 1;
 
+// MTLStorageMode. For UI atlases + sprites the CPU needs to write pixels
+// and the GPU needs to sample — `.shared` is the safe default. On iOS-sim
+// under Apple Silicon the convenience class method's default storage
+// isn't reliably shared, so we set it explicitly in metal_create_texture_ios.
+MTL_STORAGE_MODE_SHARED :u64: 0;
+
 // MTLPrimitiveType.
 MTL_PRIMITIVE_TYPE_TRIANGLE :u64: 3;
 
@@ -84,11 +90,18 @@ MetalGPU :: struct {
 }
 
 impl GPU for MetalGPU {
+    // Two-phase init: callers can `init(null, 0, 0)` first to allocate
+    // device + queue eagerly (lets the UI pipeline compile shaders before
+    // UIKit hands us a layer), then re-call `init(layer, w, h)` once the
+    // CAMetalLayer is available. The second call only updates the layer
+    // ref + dims; device/queue are preserved.
     init :: (self: *MetalGPU, target: *void, pixel_w: s32, pixel_h: s32) -> bool {
         inline if OS != .ios { return false; }
-        self.layer   = target;
-        self.pixel_w = pixel_w;
-        self.pixel_h = pixel_h;
+        if target != null {
+            self.layer   = target;
+            self.pixel_w = pixel_w;
+            self.pixel_h = pixel_h;
+        }
         metal_init_ios(self);
     }
 
@@ -200,12 +213,19 @@ impl GPU for MetalGPU {
 // so non-iOS builds never reference the unresolved Metal symbols below.
 // ───────────────────────────────────────────────────────────────────────────
 
+// init() may be called twice: once with target==null to create device +
+// queue eagerly (so the UI pipeline can compile shaders before UIKit
+// has a layer for us), then again with target=CAMetalLayer once
+// `-[SxAppDelegate didFinishLaunching:]` has installed the view.
+// Both calls go through this helper; it's idempotent on the device/queue
+// and only touches the layer when one's been supplied.
 metal_init_ios :: (self: *MetalGPU) -> bool {
     inline if OS != .ios { return false; }
-    if self.layer == null { return false; }
 
-    self.device = MTLCreateSystemDefaultDevice();
-    if self.device == null { return false; }
+    if self.device == null {
+        self.device = MTLCreateSystemDefaultDevice();
+        if self.device == null { return false; }
+    }
 
     msg_oo    : (*void, *void, *void)  -> void  = xx objc_msgSend;
     msg_ou    : (*void, *void, u64)    -> void  = xx objc_msgSend;
@@ -213,15 +233,19 @@ metal_init_ios :: (self: *MetalGPU) -> bool {
     msg_osize : (*void, *void, CGSize) -> void  = xx objc_msgSend;
     msg_o     : (*void, *void)         -> *void = xx objc_msgSend;
 
-    msg_oo(self.layer, sel_registerName("setDevice:".ptr), self.device);
-    msg_ou(self.layer, sel_registerName("setPixelFormat:".ptr), MTL_PIXEL_FORMAT_BGRA8_UNORM);
-    msg_ob(self.layer, sel_registerName("setFramebufferOnly:".ptr), 1);
+    if self.queue == null {
+        self.queue = msg_o(self.device, sel_registerName("newCommandQueue".ptr));
+        if self.queue == null { return false; }
+    }
 
-    size := CGSize.{ width = xx self.pixel_w, height = xx self.pixel_h };
-    msg_osize(self.layer, sel_registerName("setDrawableSize:".ptr), size);
+    if self.layer != null {
+        msg_oo(self.layer, sel_registerName("setDevice:".ptr), self.device);
+        msg_ou(self.layer, sel_registerName("setPixelFormat:".ptr), MTL_PIXEL_FORMAT_BGRA8_UNORM);
+        msg_ob(self.layer, sel_registerName("setFramebufferOnly:".ptr), 1);
 
-    self.queue = msg_o(self.device, sel_registerName("newCommandQueue".ptr));
-    if self.queue == null { return false; }
+        size := CGSize.{ width = xx self.pixel_w, height = xx self.pixel_h };
+        msg_osize(self.layer, sel_registerName("setDrawableSize:".ptr), size);
+    }
 
     true;
 }
@@ -457,6 +481,12 @@ metal_create_texture_ios :: (self: *MetalGPU, w: s32, h: s32, format: TextureFor
         pixel_format, xx w, xx h, 0);
     if desc == null { return 0; }
 
+    // Force shared storage so the CPU can keep writing pixels (atlas updates,
+    // sprite uploads). On iOS-sim under Apple Silicon the convenience class
+    // method's default storage isn't reliably shared for every format.
+    msg_ou_void : (*void, *void, u64) -> void = xx objc_msgSend;
+    msg_ou_void(desc, sel_registerName("setStorageMode:".ptr), MTL_STORAGE_MODE_SHARED);
+
     msg_oo : (*void, *void, *void) -> *void = xx objc_msgSend;
     tex := msg_oo(self.device, sel_registerName("newTextureWithDescriptor:".ptr), desc);
     if tex == null { return 0; }