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.

src/ir/emit_llvm.zig

@@ -2705,6 +2705,10 @@ pub const LLVMEmitter = struct {
                 if (child_info == .closure) {
                     return self.getClosureStructType();
                 }
+                // ?Protocol → protocol struct (ctx ptr = field 0 is null when none).
+                if (child_info == .@"struct" and child_info.@"struct".is_protocol) {
+                    return self.toLLVMType(opt.child);
+                }
                 // ?T → { T, i1 }
                 var field_types: [2]c.LLVMTypeRef = .{
                     self.toLLVMType(opt.child),