atomics A.0a: lib + IR ops + recognizer, emit bails (lock commit)

Stream A (atomics) foundation. Net-new atomic load/store codegen path, wired
end-to-end except LLVM emission, which deliberately bails loudly so the example
locks to a clean diagnostic (A.0b turns it green — cadence: no commit both adds a
test and makes it pass).

- library/modules/std/atomic.sx: Ordering enum, Atomic($T) transparent wrapper
  (init/load/store, seq_cst-only for now), atomic_load/atomic_store #builtin
  intrinsics. Opt-in import, NOT in the universal std facade (Ordering in the
  prelude grows every program's type table + churns 37 .ir snapshots).
- IR: atomic_load/atomic_store ops + AtomicOrdering (all 5) + structs (inst.zig);
  print arms; comptime_vm arms reuse load/store (single-thread correct);
  recognizer tryLowerAtomicIntrinsic (const-ordering + scalar-size guards, both
  loud); emit dispatch -> emitAtomicLoad/Store bail via comptime_failed.
- examples/1700-atomics-load-store.sx locked to the bail diagnostic.

Full ordering surface (a.load(.acquire)) blocked on comptime-constant ordering
propagation (comptime enum value params) — A.0.5, migrated not legacy.

src/ir/lower.zig

@@ -1836,6 +1836,7 @@ pub const Lowering = struct {
     pub const hasCastWithRuntimeType = lower_call.hasCastWithRuntimeType;
     pub const lowerRuntimeDispatchCall = lower_call.lowerRuntimeDispatchCall;
     pub const tryLowerReflectionCall = lower_call.tryLowerReflectionCall;
+    pub const tryLowerAtomicIntrinsic = lower_call.tryLowerAtomicIntrinsic;
     pub const reflectionArgIsType = lower_call.reflectionArgIsType;
     pub const reflectionTypeArgGuard = lower_call.reflectionTypeArgGuard;
     pub const reflectionErrorSentinel = lower_call.reflectionErrorSentinel;