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.

library/modules/ui/dock.sx

@@ -457,13 +457,11 @@ impl View for DockPanel {
 // Dock — dockable container with drag-and-drop zones
 // =============================================================================
 
-// Global delta_time pointer — set by main.sx
-g_dock_delta_time : *f32 = null;
-
 Dock :: struct {
     children: List(ViewChild);
     alignments: List(Alignment);
     interaction: *DockInteraction;  // heap-allocated, shared with DockPanels
+    delta_time: *f32;
 
     // Config
     background: ?Color;
@@ -475,11 +473,12 @@ Dock :: struct {
     enable_corners: bool;
     on_dock: ?Closure(s64, DockZone);
 
-    make :: (interaction: *DockInteraction) -> Dock {
+    make :: (interaction: *DockInteraction, delta_time: *f32) -> Dock {
         d : Dock = ---;
         d.children = List(ViewChild).{};
         d.alignments = List(Alignment).{};
         d.interaction = interaction;
+        d.delta_time = delta_time;
         d.background = null;
         d.corner_radius = 0.0;
         d.hint_size = 40.0;
@@ -519,8 +518,7 @@ impl View for Dock {
         interaction := self.interaction;
         interaction.ensure_capacity(self.children.len);
 
-        // Tick animations (g_dock_delta_time is always set before main loop)
-        dt : f32 = g_dock_delta_time.*;
+        dt : f32 = self.delta_time.*;
         interaction.tick_animations(dt);
 
         i : s64 = 0;