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/ui/pipeline.sx

@@ -1,6 +1,7 @@
 #import "modules/std.sx";
 #import "modules/allocators.sx";
 #import "modules/opengl.sx";
+#import "modules/gpu/api.sx";
 #import "modules/ui/types.sx";
 #import "modules/ui/render.sx";
 #import "modules/ui/events.sx";
@@ -24,6 +25,23 @@ UIPipeline :: struct {
     has_body: bool;
     parent_allocator: Allocator;
 
+    // GPU protocol backend. When `has_gpu`, the pipeline propagates this
+    // to its renderer + font, and skips the per-frame GL state setup in
+    // commit_gpu (Metal bakes blend mode into the pipeline state).
+    gpu: GPU = ---;
+    has_gpu: bool = false;
+
+    // Set the GPU dispatch BEFORE calling init() / init_font() so the
+    // shaders + atlas land on the right backend.
+    set_gpu :: (self: *UIPipeline, gpu: GPU) {
+        self.gpu = gpu;
+        self.has_gpu = true;
+        self.renderer.gpu = gpu;
+        self.renderer.has_gpu = true;
+        self.font.gpu = gpu;
+        self.font.has_gpu = true;
+    }
+
     init :: (self: *UIPipeline, width: f32, height: f32) {
         self.render_tree = RenderTree.init();
         self.renderer.init();
@@ -149,14 +167,18 @@ UIPipeline :: struct {
     }
 
     commit_gpu :: (self: *UIPipeline) {
-        glEnable(GL_BLEND);
-        glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
-        glDisable(GL_DEPTH_TEST);
+        if !self.has_gpu {
+            glEnable(GL_BLEND);
+            glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
+            glDisable(GL_DEPTH_TEST);
+        }
 
         self.renderer.begin(self.screen_width, self.screen_height, self.font.texture_id);
         self.renderer.process(@self.render_tree);
         self.renderer.flush();
 
-        glDisable(GL_BLEND);
+        if !self.has_gpu {
+            glDisable(GL_BLEND);
+        }
     }
 }