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/backend/llvm/ops.zig

@@ -15,6 +15,8 @@ const FieldAccess = ir_inst.FieldAccess;
 const EnumInit = ir_inst.EnumInit;
 const Subslice = ir_inst.Subslice;
 const Store = ir_inst.Store;
+const AtomicLoad = ir_inst.AtomicLoad;
+const AtomicStore = ir_inst.AtomicStore;
 const Conversion = ir_inst.Conversion;
 const GlobalId = ir_inst.GlobalId;
 const GlobalSet = ir_inst.GlobalSet;
@@ -363,6 +365,29 @@ pub const Ops = struct {
         self.e.advanceRefCounter();
     }
 
+    // ── Atomics ───────────────────────────────────────────
+    // A.0a (Stream A) lock: the IR ops, lowering, and comptime VM are wired,
+    // but LLVM emission deliberately BAILS LOUDLY (clean diagnostic + build
+    // abort via `comptime_failed`) rather than silently emitting a non-atomic
+    // load/store. A.0b replaces these bodies with the real builders:
+    //   load:  LLVMBuildLoad2 + LLVMSetOrdering + LLVMSetAlignment
+    //   store: LLVMBuildStore  + LLVMSetOrdering + LLVMSetAlignment
+    // (ordering via an explicit sx-tag → LLVMAtomicOrdering switch).
+    pub fn emitAtomicLoad(self: Ops, instruction: *const Inst, a: AtomicLoad) void {
+        _ = a;
+        std.debug.print("error: atomic load LLVM emission not yet implemented (Stream A, A.0b)\n", .{});
+        self.e.comptime_failed = true;
+        // Keep emit from crashing downstream: yield an undef of the result type.
+        self.e.mapRef(c.LLVMGetUndef(self.e.toLLVMType(if (instruction.ty == .void) .i64 else instruction.ty)));
+    }
+
+    pub fn emitAtomicStore(self: Ops, a: AtomicStore) void {
+        _ = a;
+        std.debug.print("error: atomic store LLVM emission not yet implemented (Stream A, A.0b)\n", .{});
+        self.e.comptime_failed = true;
+        self.e.advanceRefCounter();
+    }
+
     // ── Globals ───────────────────────────────────────────
     pub fn emitGlobalGet(self: Ops, instruction: *const Inst, gid: GlobalId) void {
         const llvm_global = self.e.global_map.get(gid.index()) orelse {