issue-0028: ?Protocol = null sentinel-shaped optional protocols

Protocol structs registered via registerProtocolDecl carry a new
is_protocol flag; the ?T paths in sizeOf/typeSizeBytes/toLLVMType
recognise it and lay out ?Protocol as the protocol struct itself
(ctx == null IS the "none" state), matching how ?Closure / ?*T are
sentinel-shaped — no extra storage.

Method dispatch on ?Protocol auto-unwraps in lowerCall's field-access
path; the unwrap is structurally a no-op so we just rebind obj_ty to
the payload type. resolveCallParamTypes extended for optional-protocol
receivers so enum-literal args (gpu.create_texture(.r8, ...)) get the
right target_type and don't silently collapse to tag=0 : s32 — same
issue-0031-class bug closed in Session 66, one type-system layer
deeper.

Library: UIRenderer / UIPipeline / GlyphCache migrated from the verbose
gpu: GPU = ---; has_gpu: bool pattern to gpu: ?GPU = null. set_gpu no
longer maintains a parallel bool flag.

Bundled: dock.sx threads delta_time as a struct field rather than via
a global pointer (cleanup unrelated to issue-0028, committed alongside).

Verified: 85/85 regression tests pass; iOS-sim chess + macOS chess
both render correctly post-migration.

examples/98-optional-protocol.sx

@@ -0,0 +1,34 @@
+// `?Protocol = null` — optional protocol boxes use sentinel-shape
+// (ctx == null is the "none" state), so they cost no extra storage
+// beyond the protocol's standard 2-pointer layout. Method calls on
+// a non-null optional protocol auto-unwrap and dispatch through the
+// vtable / inline fn-ptrs as usual.
+
+#import "modules/std.sx";
+
+GPU :: protocol {
+  ping :: () -> s64;
+}
+
+Impl :: struct {}
+impl GPU for Impl {
+  ping :: (self: *Impl) -> s64 { 42; }
+}
+
+main :: () -> s32 {
+  g : ?GPU = null;
+  if g != null {
+    print("BAD: g not null at start\n");
+  } else {
+    print("g initially null\n");
+  }
+
+  g = xx @Impl.{};
+  if g != null {
+    n := g.ping();
+    print("after assign: g.ping() = {}\n", n);
+  } else {
+    print("BAD: g still null after assign\n");
+  }
+  0;
+}

examples/issue-0028.sx

@@ -1,53 +0,0 @@
-// issue-0028: Feature — make protocol boxes assignable to an optional
-// type so callers can spell "no GPU bound" as `?GPU = null` instead of
-// the verbose `T = ---; has_T: bool` pattern.
-//
-// ── Current pattern (verbose) ─────────────────────────────────────────────
-//
-//   gpu: GPU = ---;
-//   has_gpu: bool = false;
-//   ...
-//   if self.has_gpu { self.gpu.create_shader(...); }
-//
-// ── Proposed pattern ──────────────────────────────────────────────────────
-//
-//   gpu: ?GPU = null;
-//   ...
-//   if self.gpu != null { self.gpu.create_shader(...); }
-//
-// ── Where the verbose pattern lives today ─────────────────────────────────
-//
-// library/modules/ui/renderer.sx     — UIRenderer.gpu + has_gpu
-// library/modules/ui/glyph_cache.sx  — GlyphCache.gpu + has_gpu
-// library/modules/ui/pipeline.sx     — UIPipeline.gpu + has_gpu (+ set_gpu)
-// library/modules/platform/uikit.sx  — UIKitPlatform.frame_closure +
-//                                       has_frame_closure (Closure type,
-//                                       same pattern but on a closure)
-//
-// ── Implementation sketch ─────────────────────────────────────────────────
-//
-// Protocol boxes are 2-pointer structs ({vtable, ctx} or {ctx, fn_ptrs...}
-// depending on the inline-vs-vtable shape — see src/ir/lower.zig
-// `buildProtocolValue` ~7800-7869). `?T` for these can use `vtable_ptr ==
-// null` (or `ctx == null`, depending on layout choice) as the "none"
-// sentinel — no extra storage needed. This matches the existing
-// optional-closure handling at src/ir/emit_llvm.zig where `?Closure` uses
-// `fn_ptr == null` as none.
-//
-// Approach:
-//   1. Extend `?T` type construction to accept T being a protocol type.
-//      Files: src/ir/types.zig + src/ir/lower.zig (type-resolution).
-//   2. Implement `optional_wrap` / `optional_unwrap` /
-//      `optional_has_value` for protocol-typed payloads in
-//      src/ir/emit_llvm.zig — model after the closure-optional path.
-//   3. Keep the existing `T = ---; has_T: bool` pattern working — the
-//      new `?T` is additive, not a replacement. Don't churn existing
-//      files (uikit.sx's frame_closure pattern stays).
-//
-// ── Syntax constraint ─────────────────────────────────────────────────────
-//
-// `?T` syntax already exists for primitives + pointers. Extending to
-// protocols is a type-system change; no new surface syntax needed.
-
-#import "modules/std.sx";
-main :: () -> s32 { 0; }