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`.

specs.md

@@ -1061,6 +1061,45 @@ main :: () {
 }
 ```
 
+#### Default Parameter Values
+
+A parameter can declare a default value with `name: type = expr`. When a
+caller omits the trailing positional argument, the compiler substitutes
+the default expression at the call site:
+
+```sx
+greet :: (name: string, prefix: string = "Hello") {
+    print("{} {}!\n", prefix, name);
+}
+
+greet("world");                 // prints "Hello world!"
+greet("world", "Good morning"); // prints "Good morning world!"
+```
+
+The default expression is captured as an AST node at parse time and
+re-lowered fresh at each call site, so runtime expressions like
+`context.allocator` resolve in the **caller's** scope, not the callee's
+definition site. This is the mechanism that lets stdlib containers like
+`List(T)` expose an optional allocator argument that defaults to
+`context.allocator` without requiring callers to thread one through:
+
+```sx
+// In std.sx:
+List :: struct ($T: Type) {
+    append :: (list: *List(T), item: T, alloc: Allocator = context.allocator) {
+        // ... grows via `alloc.alloc(...)` ...
+    }
+}
+
+// Call sites:
+list.append(42);                         // alloc = current context.allocator
+list.append(42, self.parent_allocator);  // alloc = the named long-lived owner
+```
+
+Defaults are only consulted for **trailing** missing positional args; once
+a position is provided, all earlier positions must also be provided. There
+is no named-argument syntax for skipping middle defaults.
+
 ### Enum Definition
 
 ```sx
@@ -2041,7 +2080,7 @@ struct_decl     = IDENT '::' 'struct' '{' struct_member* '}'
 struct_member   = field_group | '#using' IDENT ';'
 field_group     = IDENT (',' IDENT)* ':' type ('=' expr)? ';'
 params          = param (',' param)* ','?
-param           = IDENT ':' type
+param           = IDENT ':' type ('=' expr)?
 block           = '{' stmt* '}'
 stmt            = decl | assignment ';' | multi_assign ';' | return_stmt | defer_stmt | insert_stmt
                 | push_stmt | break_stmt | continue_stmt | expr ';'