mem: List(T) mutations gain optional alloc: Allocator = context.allocator

The chess panel-text regression (text vanished after the first move on
macOS) had a single root cause: GlyphCache's entries List, hash table,
and shaped_buf grew through `context.allocator` — which during render
is the per-frame arena. On the next arena reset the backing died, and
subsequent glyph lookups read garbage / wrote into freshly-allocated
view-tree memory.

Fix is shaped as the user proposed: `List(T)`'s mutations take an
optional trailing `alloc: Allocator = context.allocator` argument. No
allocator stored on the container, no init ceremony, every existing
`list.append(item)` callsite keeps working unchanged. Long-lived
owners now write `list.append(item, self.parent_allocator)` and the
arena-leak bug becomes impossible to write accidentally.

Default-arg substitution previously only fired for identifier callees
(`expandCallDefaults` at lower.zig:7978). Extended to the generic
struct-method dispatch path (`list.append(...)` lands here) via a new
`appendDefaultArgs` helper that lowers fd.params[i].default_expr in
the caller's scope and appends to the lowered args slice.

Long-lived owners updated to capture `parent_allocator: Allocator` at
init and use it for every internal growth:

- GlyphCache (the chess bug) — entries, shaped_buf, hash_keys,
  hash_vals, atlas bitmap.
- DockInteraction — drops the existing `push Context` workaround in
  `ensure_capacity` for the explicit-arg form.
- StateStore — entries list + per-entry data buffer.
- Gles3Gpu, MetalGPU — shaders, buffers, textures (atlas-grow during
  render would otherwise leak resources into the frame arena).

Also kept: an operator-precedence fix in pipeline.sx
(`(self.frame_index & 1) == 0` instead of
`self.frame_index & 1 == 0`, which parses as
`self.frame_index & (1 == 0)` = always 0). That was a stealth
single-arena-only bug that masked the GlyphCache one for a long time.

Docs:
- specs.md §11 documents `param: T = expr` default parameter values.
  The parser already supported it — formalised in the spec now.
- current/CHECKPOINT-MEM.md logs the change.
- CLAUDE.md REJECTED PATTERNS gains a "Long-lived containers growing
  through context.allocator" section with the `parent_allocator`
  capture template and the list of existing examples to mirror.

155/155 example tests pass — zero-diff against snapshots since every
existing callsite still resolves to `context.allocator`.

library/modules/ui/state.sx

@@ -24,10 +24,12 @@ StateEntry :: struct {
 StateStore :: struct {
     entries: List(StateEntry);
     current_generation: s64;
+    parent_allocator: Allocator;
 
     init :: (self: *StateStore) {
         self.entries = List(StateEntry).{};
         self.current_generation = 0;
+        self.parent_allocator = context.allocator;
     }
 
     get_or_create :: (self: *StateStore, id: s64, $T: Type, default: T) -> State(T) {
@@ -42,14 +44,14 @@ StateStore :: struct {
         }
 
         // Create new entry
-        data : [*]u8 = xx context.allocator.alloc(size_of(T));
+        data : [*]u8 = xx self.parent_allocator.alloc(size_of(T));
         memcpy(data, @default, size_of(T));
         self.entries.append(.{
             id = id,
             data = data,
             size = size_of(T),
             generation = self.current_generation
-        });
+        }, self.parent_allocator);
         State(T).{ ptr = xx data };
     }