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/68-generic-protocol-constraint.sx

@@ -0,0 +1,76 @@
+// Generic struct `Animated($T: Lerpable)` monomorphized with a struct type — the
+// `#inline` protocol constraint participates in method dispatch via `self.from.lerp(...)`.
+
+#import "modules/std.sx";
+#import "modules/math";
+
+Lerpable :: protocol #inline {
+    lerp :: (b: Self, t: f32) -> Self;
+}
+
+Size :: struct {
+    width, height: f32;
+    zero :: () -> Size => .{ width = 0.0, height = 0.0 };
+}
+
+impl Lerpable for Size {
+    lerp :: (self: Size, b: Size, t: f32) -> Size {
+        Size.{ width = self.width + (b.width - self.width) * t,
+               height = self.height + (b.height - self.height) * t };
+    }
+}
+
+Animated :: struct ($T: Lerpable) {
+    current: T;
+    from: T;
+    to: T;
+    elapsed: f32;
+    duration: f32;
+    active: bool;
+
+    make :: (value: T) -> Animated(T) {
+        Animated(T).{
+            current = value, from = value, to = value,
+            elapsed = 0.0, duration = 0.0, active = false
+        };
+    }
+
+    set_immediate :: (self: *Animated(T), value: T) {
+        self.current = value;
+        self.from = value;
+        self.to = value;
+        self.active = false;
+    }
+
+    animate_to :: (self: *Animated(T), target: T, dur: f32) {
+        self.from = self.current;
+        self.to = target;
+        self.elapsed = 0.0;
+        self.duration = dur;
+        self.active = true;
+    }
+
+    tick :: (self: *Animated(T), dt: f32) {
+        if !self.active { return; }
+        self.elapsed += dt;
+        t := clamp(self.elapsed / self.duration, 0.0, 1.0);
+        self.current = self.from.lerp(self.to, t);
+        if t >= 1.0 {
+            self.current = self.to;
+            self.active = false;
+        }
+    }
+}
+
+main :: () -> void {
+    anim := Animated(Size).make(Size.zero());
+    anim.set_immediate(Size.{ width = 100.0, height = 50.0 });
+    print("after set: {}x{}\n", anim.current.width, anim.current.height);
+
+    anim.animate_to(Size.{ width = 200.0, height = 100.0 }, 1.0);
+    anim.tick(0.5);
+    print("mid anim: {}x{}\n", anim.current.width, anim.current.height);
+
+    anim.tick(0.5);
+    print("end anim: {}x{}\n", anim.current.width, anim.current.height);
+}