checkpoint: M4.0 + M4.A complete; M4.B remaining

Snapshot of FFI progress mid-M4. Allocator-aware sx-defined-class
lifecycle is done end-to-end (state struct + +alloc + -dealloc).
Stdlib NSObject + autoreleasepool helper landed; defer-release
pattern works at user code. Property ARC ops (M4.B) is the next
slice.

185/185 example tests pass; chess on iOS-sim regression-verified
at every M4 sub-commit.

current/CHECKPOINT-FFI.md

@@ -221,10 +221,55 @@ plus 2 codegen fixes surfaced along the way.**
 
 ## Current state
 
-- 169/169 example tests pass; `zig build test` green.
-- Phase 3.0/3.1/3.2 + M1.0–M1.3 + M2.1–M2.3 + M3 (full
-  retire-`uikit_register_classes`) all landed.
+- 185/185 example tests pass; `zig build test` green.
+- Phase 3.0/3.1/3.2 + M1.0–M1.3 + M2.1–M2.3 + M3 + M4.0 + M4.A all landed.
 - Chess on macOS / iOS-sim / Android all build and run.
+
+**M4.0 — context.allocator threading** (4 commits this session):
+- `__sx_allocator: Allocator` prepended at field index 0 of every
+  sx-defined class's state struct
+  ([src/ir/lower.zig:`objcDefinedStateStructType`](../src/ir/lower.zig)).
+- Sx-side `Cls.alloc()` intercepted in `lowerObjcStaticCall` for
+  sx-defined classes — emits the inline alloc-and-init sequence
+  using the caller's `current_ctx_ref`. `push Context.{ allocator =
+  arena }` now backs the next `SxFoo.alloc()`.
+- Runtime-side `+alloc` IMP is now a shim that reads
+  `__sx_default_context.allocator` and forwards to the same shared
+  helper.
+- Shared `emitObjcDefinedAllocAndInit(fcd, cls_ref, ctx_addr)` does
+  the work: class_createInstance → ctx.allocator.alloc(STATE_SIZE)
+  via the inline-protocol fn-ptr → memset 0 → store allocator at
+  state[0] → object_setIvar(__sx_state).
+- `-dealloc` IMP loads `state->__sx_allocator` and dispatches
+  `allocator.dealloc(state)` instead of the old raw `free(state)`.
+- TrackingAllocator now sees sx-defined class alloc/dealloc pairs.
+
+**M4.A — stdlib NSObject + autoreleasepool** (1 commit):
+- `NSObject :: #foreign #objc_class("NSObject")` declared in
+  std/objc.sx with the full inherited surface: retain/release/
+  autorelease/new/alloc/init/class/description/hash/isEqual_/
+  isKindOfClass_/respondsToSelector_.
+- All previously-unrooted foreign classes in uikit.sx now
+  `#extends NSObject;` (NSValue, NSNumber, NSDictionary, etc.) so
+  M2.3's extends-chain dispatch finds retain/release on any UIKit
+  type.
+- `autoreleasepool(body: Closure())` stdlib helper wraps the
+  push/defer-pop pair.
+- Canonical idiom enabled: `view := UIView.alloc().init(); defer
+  view.release();`.
+- Smoke test [examples/ffi-objc-arc-01-autoreleasepool.sx](../examples/ffi-objc-arc-01-autoreleasepool.sx)
+  exercises the retain/release + autoreleasepool round-trip.
+
+**M4.B remaining** (compiler-internal property ARC ops; not yet
+landed):
+- `objcPropertyKind(field)` helper + `ensureArcRuntimeDecls`.
+- Setter wiring strong/weak/copy ops.
+- Getter via objc_loadWeakRetained for weak properties.
+- Dealloc property-aware cleanup (release strong/copy, destroyWeak
+  weak) before freeing state struct.
+
+**Previous-session wins still in this checkpoint:**
+- M4.0 / M4.A built on top of these earlier commits this session:
 - `library/modules/platform/uikit.sx` follow-up cleanup just
   shipped: every `plat: *UIKitPlatform` helper and every
   `(self: *void, _cmd: *void, ...)` trampoline is now a method