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.

examples/1700-atomics-load-store.sx

@@ -0,0 +1,17 @@
+// Atomic($T) load/store (seq_cst), single-thread.
+// Stream A (atomics) A.0 — the first net-new atomic codegen example.
+// Explicit orderings (a.load(.acquire)) arrive in A.0.5; see PLAN-ATOMICS.md.
+// Atomics is an opt-in import (not in the universal prelude) — like `trace`.
+#import "modules/std.sx";
+#import "modules/std/atomic.sx";
+
+main :: () {
+    a := Atomic(i64).init(7);
+    print("init: {}\n", a.load());
+
+    a.store(42);
+    print("after store: {}\n", a.load());
+
+    a.store(a.load() + 1);
+    print("incremented: {}\n", a.load());
+}

examples/expected/1700-atomics-load-store.exit

@@ -0,0 +1 @@
+1

examples/expected/1700-atomics-load-store.stderr

@@ -0,0 +1,2 @@
+error: atomic load LLVM emission not yet implemented (Stream A, A.0b)
+error: atomic store LLVM emission not yet implemented (Stream A, A.0b)

examples/expected/1700-atomics-load-store.stdout

@@ -0,0 +1 @@
+