mem: implicit-context foundation + many compiler fixes

The session-long set of changes that lay the groundwork for the
Jai-literal implicit-Context-parameter refactor. Lots of accumulated
work; the new arrival is the implicit-ctx foundation (steps 1+2 of
the plan in current/CHECKPOINT-MEM.md):

  Step 1 — `CAllocator :: struct {}` stateless allocator in
    library/modules/allocators.sx, delegating directly to
    libc_malloc/libc_free. `ConstantValue` in src/ir/inst.zig gains a
    `func_ref: FuncId` leaf so nested aggregates can carry function
    pointers (the inline Allocator value's fn-ptr fields). Switch
    sites updated in emit_llvm.zig, print.zig, interp.zig.

  Step 2 — `emitDefaultContextGlobal` in src/ir/lower.zig synthesises
    a static `__sx_default_context` global with a nested-aggregate
    init_val pointing at the CAllocator → Allocator thunks. The
    second-pass `initVtableGlobals` in emit_llvm.zig is generalised
    to handle `.aggregate` init_vals (re-emits after func_map is
    populated so func_ref leaves resolve to real symbols).

Also folded in from earlier work this session:

  - Phase 1.1: `xx value` heap-copy in `buildProtocolValue` routes
    through `context.allocator` via the new `allocViaContext` helper.
  - interp.zig: `marshalForeignArg` double-offset bug fixed —
    `heapSlice` already adds `hp.offset` to the slice ptr, so the
    extra `+ hp.offset` was scribbling memcpy/memset into adjacent
    heap state, corrupting `heap.items[0]`. Symptom: `build_format`
    at comptime produced zero bytes, all `print` calls failed.
  - Lazy lowering: `lazyLowerFunction` now declares foreign-body
    functions as extern stubs in the local (comptime) module so
    cross-module foreign calls resolve.
  - Allocator API: all stdlib allocators on one-line `init() -> *T`
    (CAllocator/GPA: libc-backed; Arena/TrackingAllocator: parent-
    backed; BufAlloc: embeds state at head of user buffer).
  - issues 0038 (transitive #import), 0039 (chess + stdlib migration
    fallout), 0040 (generic struct method dot-dispatch), 0041
    (pointer types as type-arg), 0042 (alias name resolution) — all
    fixed; regression tests in examples/.
  - Diagnostic: `emitError` now embeds the lowering's
    `current_source_file` and enclosing function in the literal
    message; SX_TRACE_UNRESOLVED=1 dumps a Zig stack trace at the
    emit site so misattributed spans can't hide where the failure
    is.
  - tools/verify-step.sh (all-platforms gate) and tools/scratch.sh
    (interp/codegen parity tester) added.

Test suite: 152 example tests pass; chess builds + screenshots on
macOS / iOS sim / Android.

library/modules/ui/pipeline.sx

@@ -5,6 +5,7 @@
 #import "modules/ui/types.sx";
 #import "modules/ui/render.sx";
 #import "modules/ui/events.sx";
+#import "modules/ui/font.sx";
 #import "modules/ui/view.sx";
 #import "modules/ui/renderer.sx";
 
@@ -17,9 +18,11 @@ UIPipeline :: struct {
     root: ViewChild;
     has_root: bool;
 
-    // Frame arena infrastructure
-    arena_a: Arena;
-    arena_b: Arena;
+    // Frame arena infrastructure. Both arenas are typed `*Arena`
+    // pointers (per the init-returns-typed-pointer API in
+    // allocators.sx). Cast to Allocator at use sites via `xx arena_a`.
+    arena_a: *Arena;
+    arena_b: *Arena;
     frame_index: s64;
     body: Closure() -> View;
     has_body: bool;
@@ -65,8 +68,8 @@ UIPipeline :: struct {
         self.has_body = true;
         self.parent_allocator = context.allocator;
         // Initialize both arenas (256KB initial, grows automatically)
-        self.arena_a.create(self.parent_allocator, 262144);
-        self.arena_b.create(self.parent_allocator, 262144);
+        self.arena_a = Arena.init(self.parent_allocator, 262144);
+        self.arena_b = Arena.init(self.parent_allocator, 262144);
         self.frame_index = 0;
     }
 
@@ -129,7 +132,7 @@ UIPipeline :: struct {
     }
 
     tick_with_body :: (self: *UIPipeline) {
-        build_arena : *Arena = if self.frame_index & 1 == 0 then @self.arena_a else @self.arena_b;
+        build_arena : *Arena = if self.frame_index & 1 == 0 then self.arena_a else self.arena_b;
         build_arena.reset();
 
         // Reset render_tree nodes (backing is stale after arena reset)