atomics A.3b: real swap (xchg) + fence emission + unit test (green)

emitAtomicRmw xchg arm (swap) and emitAtomicFence (LLVMBuildFence) now real.
examples/1703 (swap old=7/now=42, 'atomicrmw xchg') + 1704 (fence release/acquire/
seq_cst) green. Unit test 'emit: atomic swap (xchg) + fence'. Stream A
(atomics) is feature-complete: load/store, RMW (add/sub/and/or/xor/min/max),
compare_exchange[_weak], swap, fence. Suite green (721/0).

current/CHECKPOINT-ATOMICS.md

@@ -124,3 +124,9 @@ follow-up, not an atomics blocker.
 - **A.1** — RMW: atomic_rmw op + RmwKind + recognizer (rmwKindFromName, integer-only) +
   7 fetch_* methods/intrinsics. A.1a bail-lock → A.1b real LLVMBuildAtomicRMW (signed|unsigned
   min/max). comptime_vm real RMW. 1701 + unit test. Suite green (716/0).
+- **A.2** — CAS: atomic_cmpxchg op + recognizer (dual-ordering validation) + emit (?T from
+  {actual,!success}) + comptime VM. compare_exchange/_weak methods. examples 1702 + 1186.
+  Review agent died; self-verified comptime↔runtime agreement, sub-8, ordering edges.
+  (Commits dca396e/79895be; A.2 has_value fix folded into A.3a.)
+- **A.3** — swap (atomicrmw xchg) + fence (new atomic_fence op). A.3a bail-lock → A.3b real.
+  examples 1703/1704/1187 + unit test. Stream A feature-complete. Suite green (721/0).