metal: GPU protocol + MetalGPU renders MSL triangle on iOS

Phase 8 step 3a of the Metal renderer port:

- New library/modules/gpu/ with types.sx (handles + ClearColor +
  TextureFormat enum), api.sx (GPU :: protocol { ... } covering the
  lifecycle / per-frame / resource / per-draw surface), and metal.sx
  (MetalGPU backend implementing the protocol against CAMetalLayer).
  Resource handles are 1-based indices into backend List(*void) tables.
  MTL aggregates >16 bytes (MTLRegion, MTLScissorRect) pass via *T to
  match arm64 Apple's indirect-by-reference ABI; MTLClearColor + CGSize
  go through the HFA path as direct fn-pointer casts on objc_msgSend.

- UIKitPlatform got a gpu_mode: GpuMode toggle + sibling SxMetalView
  class registration. In metal mode init skips EAGL context, the
  did_finish_launching IMP skips the EAGL drawable-properties dict,
  layoutSubviews reads the layer's bounds * dpi_scale into pixel_w/h
  instead of allocating a GL renderbuffer, and end_frame is a no-op
  (the MetalGPU owns its own present).

- examples/63-metal-clear.sx verifies the pipeline end-to-end on iOS
  sim — compiles a pass-through MSL shader (packed_float2/packed_float4
  to avoid alignment padding), uploads 3 vertices, draws a colored
  triangle on a dark-blue clear.

Compiler fixes (filed-and-fixed in this branch):

- inline if X { return E; } followed by a fall-through final expression
  no longer emits two terminators into the same basic block. Verified
  by examples/83-inline-if-return-fallthrough.sx.

- Top-level type alias Name :: u32; now resolves correctly as the type
  annotation on a global variable (was treated as ptr {}, breaking
  comparisons + initializers). Verified by examples/84-global-type-alias.sx.

Issue->feature promotion:

- 16 historical examples/issue-NNNN.sx repros now confirmed-fixed and
  renamed to focused feature names (67-82). Each gains a
  tests/expected/*.txt + .exit pair so the regression suite covers them.

- 5 stale issue repros deleted (subsumed by broader tests).

Regression suite: 68 passing, 0 failed. macOS chess builds + runs; wasm
chess builds; iOS sim GLES chess still renders the full board; iOS sim
Metal demo renders the triangle.

examples/72-protocol-in-wrapper-struct.sx

@@ -0,0 +1,48 @@
+// Protocol value as a field of a wrapper struct, constructed from a stack
+// local inside a function and appended to a `List`. The payload must be
+// heap-copied so dispatch survives the constructing function returning.
+
+#import "modules/std.sx";
+
+Sizable :: protocol {
+    size :: () -> s64;
+}
+
+Widget :: struct { value: s64; }
+impl Sizable for Widget {
+    size :: (self: *Widget) -> s64 { self.value; }
+}
+
+// Wrapper struct with a protocol field (like ViewChild)
+Item :: struct {
+    view: Sizable;
+}
+
+Container :: struct {
+    items: List(Item);
+
+    add :: (self: *Container, w: Widget) {
+        p := w;  // local copy
+        self.items.append(Item.{ view = p });  // protocol created from stack local `p`
+
+        // Works here: stack local `p` is still alive
+        out("inside add: ");
+        print("{}\n", self.items.items[self.items.len - 1].view.size());
+    }
+}
+
+main :: () -> void {
+    c : Container = .{};
+    c.add(Widget.{ value = 42 });
+    c.add(Widget.{ value = 99 });
+
+    // BUG: items[0] should return 42, but returns 99 (reads items[1]'s stack slot)
+    // Both protocol values point to the same stack address (the `p` local in add())
+    r0 := c.items.items[0].view.size();
+    r1 := c.items.items[1].view.size();
+    print("items[0] = {} (expected 42)\n", r0);
+    print("items[1] = {} (expected 99)\n", r1);
+
+    // With more stack activity between add() and the reads, this crashes
+    // (stack memory overwritten by other function calls)
+}