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.

tests/expected/67-impl-for-builtin.exit

@@ -0,0 +1 @@
+0

tests/expected/67-impl-for-builtin.txt

@@ -0,0 +1,2 @@
+lerp(0, 10, 0.5) = 5.000000
+lerp(0, 10, 0.25) = 2.500000

tests/expected/68-generic-protocol-constraint.exit

@@ -0,0 +1 @@
+0

tests/expected/68-generic-protocol-constraint.txt

@@ -0,0 +1,3 @@
+after set: 100.000000x50.000000
+mid anim: 150.000000x75.000000
+end anim: 200.000000x100.000000

tests/expected/69-optional-all-null.exit

@@ -0,0 +1 @@
+0

tests/expected/69-optional-all-null.txt

@@ -0,0 +1,3 @@
+r1 = 150.000000
+r2 = 150.000000
+r3 = 200.000000

tests/expected/70-optional-roundtrip.exit

@@ -0,0 +1 @@
+0

tests/expected/70-optional-roundtrip.txt

@@ -0,0 +1,10 @@
+=== Direct calls ===
+d1 = 150.000000
+d2 = 150.000000
+d3 = 200.000000
+d4 = 110.000000
+=== Protocol dispatch ===
+r1 = 150.000000
+r2 = 150.000000
+r3 = 200.000000
+r4 = 110.000000

tests/expected/71-int-cmp-in-float-ternary.exit

@@ -0,0 +1 @@
+0

tests/expected/71-int-cmp-in-float-ternary.txt

@@ -0,0 +1,2 @@
+ok
+result = 1.000000

tests/expected/72-protocol-in-wrapper-struct.exit

@@ -0,0 +1 @@
+0

tests/expected/72-protocol-in-wrapper-struct.txt

@@ -0,0 +1,4 @@
+inside add: 42
+inside add: 99
+items[0] = 42 (expected 42)
+items[1] = 99 (expected 99)

tests/expected/73-protocol-list-from-fn.exit

@@ -0,0 +1 @@
+0

tests/expected/73-protocol-list-from-fn.txt

@@ -0,0 +1,7 @@
+=== Created in main ===
+first:  42 (expected 42)
+second: 42 (expected 42)
+=== Created in add() ===
+first:  99 (expected 99)
+second: 99 (expected 99)
+=== OK ===

tests/expected/74-protocol-dispatch-via-fn-arg.exit

@@ -0,0 +1 @@
+0

tests/expected/74-protocol-dispatch-via-fn-arg.txt

@@ -0,0 +1,8 @@
+=== Direct 1 ===
+r1 = 42
+=== Direct 2 ===
+r2 = 42
+=== From function ===
+dispatch_fn: about to dispatch
+dispatch_fn: result = 42
+=== OK ===

tests/expected/75-push-context-with-arena.exit

@@ -0,0 +1 @@
+0

tests/expected/75-push-context-with-arena.txt

@@ -0,0 +1,2 @@
+inside push
+after push

tests/expected/76-closure-returning-protocol.exit

@@ -0,0 +1 @@
+0

tests/expected/76-closure-returning-protocol.txt

@@ -0,0 +1,2 @@
+direct call works
+closure call works

tests/expected/77-list-items-assign-big-T.exit

@@ -0,0 +1 @@
+0

tests/expected/77-list-items-assign-big-T.txt

@@ -0,0 +1,4 @@
+size_of BigNode = 40
+before: sentinel = 3735928559
+after:  sentinel = 3735928559
+OK

tests/expected/78-global-compound-assign.exit

@@ -0,0 +1 @@
+0

tests/expected/78-global-compound-assign.txt

@@ -0,0 +1,10 @@
+--- Test 1: += (broken) ---
+Expected: 1, 2, 3
+counter=1
+counter=2
+counter=3
+--- Test 2: = x + 1 (works) ---
+Expected: 2, 3, 4
+counter=4
+counter=5
+counter=6

tests/expected/79-global-array-init.exit

@@ -0,0 +1 @@
+0

tests/expected/79-global-array-init.txt

@@ -0,0 +1,2 @@
+VALS: -2 -1 42 99 
+PASS

tests/expected/80-dot-shorthand-protocol-field.exit

@@ -0,0 +1 @@
+0

tests/expected/80-dot-shorthand-protocol-field.txt

@@ -0,0 +1,3 @@
+StackA: draw=42
+StackB: draw=25
+OK

tests/expected/81-global-struct-defaults.exit

@@ -0,0 +1 @@
+0

tests/expected/81-global-struct-defaults.txt

@@ -0,0 +1,6 @@
+local         running=true x=42 name=default
+g_empty       running=true x=42 name=default
+g_partial     running=true x=99 name=default
+g_override    running=false x=42 name=default
+g_reorder     running=false x=7 name=hi
+g_positional  running=false x=13 name=pos

tests/expected/82-xx-target-in-field-assign.exit

@@ -0,0 +1 @@
+0

tests/expected/82-xx-target-in-field-assign.txt

@@ -0,0 +1,2 @@
+dpi=2.000000
+delta=0.500000 fc.a=1.000000

tests/expected/83-inline-if-return-fallthrough.exit

@@ -0,0 +1 @@
+1

tests/expected/83-inline-if-return-fallthrough.txt

@@ -0,0 +1 @@
+

tests/expected/84-global-type-alias.exit

@@ -0,0 +1 @@
+0

tests/expected/84-global-type-alias.txt

@@ -0,0 +1 @@
+