checkpoint: M4 complete (M4.0 + M4.A + M4.B all landed)

12 commits this session shipped the entire M4 milestone. sx-defined
Obj-C classes now honor `context.allocator` end-to-end, route through
NSObject's retain/release at the source-language level, and emit
correct ARC ops in property setters/getters/dealloc per the Apple
ABI contract.

189/189 example tests pass; chess on iOS-sim green throughout.
Ready for M5 (closure↔block bridge) or M1.1.b (Class(T) phantom
typing) next session.

current/CHECKPOINT-FFI.md

@@ -260,13 +260,25 @@ plus 2 codegen fixes surfaced along the way.**
 - 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.
+**M4.B — property ARC ops** (4 commits, done):
+- `objcPropertyKind(field)` + `ensureArcRuntimeDecls` helpers. The
+  kind helper validates `#property(...)` modifiers and emits loud
+  diagnostics for: unknown modifier names, conflicting modifiers,
+  weak/copy on non-object fields, `#property(strong)` on `*void`.
+- Setter emits ARC ops per kind: strong → retain new + release old;
+  weak → `objc_storeWeak`; copy → `[val copy]` + release old; assign
+  → bare store.
+- Getter weak path → `objc_loadWeakRetained` + `objc_autorelease`
+  for race-safe reads. Strong/copy/assign keep the bare load.
+- Dealloc walks `#property` ivars BEFORE freeing the state struct:
+  release strong/copy ivars, destroyWeak weak ivars. Order: property
+  cleanup → state free → [super dealloc].
+
+Smoke tests `ffi-objc-arc-02-strong-property` (TrackingAllocator
+midpoint + balance) and `ffi-objc-arc-03-weak-property` (auto-nil
+after target dealloc) both pass.
+
+189/189 example tests pass; chess on iOS-sim green throughout M4.
 
 **Previous-session wins still in this checkpoint:**
 - M4.0 / M4.A built on top of these earlier commits this session: