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/inst.zig

@@ -161,6 +161,10 @@ pub const Op = union(enum) {
     load: UnaryOp, // load from pointer
     store: Store, // store value to pointer
 
+    // ── Atomics ─────────────────────────────────────────────────────
+    atomic_load: AtomicLoad, // atomic load from pointer with memory ordering
+    atomic_store: AtomicStore, // atomic store to pointer with memory ordering
+
     // ── Struct ops ──────────────────────────────────────────────────
     struct_init: Aggregate, // construct struct from field values
     struct_get: FieldAccess, // read struct field by index
@@ -294,6 +298,27 @@ pub const Store = struct {
     val_ty: TypeId = .void,
 };
 
+/// Memory ordering for atomic ops. The sx-surface `Ordering` enum
+/// (`relaxed`/`acquire`/`release`/`acq_rel`/`seq_cst`) is read statically at
+/// lower-time (the arg MUST be a constant enum literal) and baked here, so the
+/// op carries no runtime ordering operand. The LLVM mapping is EXPLICIT (LLVM's
+/// `LLVMAtomicOrdering` is non-contiguous: Monotonic=2/Acquire=4/…/SeqCst=7) —
+/// never an identity cast.
+pub const AtomicOrdering = enum { relaxed, acquire, release, acq_rel, seq_cst };
+
+pub const AtomicLoad = struct {
+    ptr: Ref,
+    ordering: AtomicOrdering,
+};
+
+pub const AtomicStore = struct {
+    ptr: Ref,
+    val: Ref,
+    /// Declared type of the stored value (same role as `Store.val_ty`).
+    val_ty: TypeId = .void,
+    ordering: AtomicOrdering,
+};
+
 pub const Conversion = struct {
     operand: Ref,
     from: TypeId,