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/print.zig

@@ -233,6 +233,11 @@ fn printInst(instruction: *const Inst, ref_idx: u32, tt: *const TypeTable, write
             try writer.print("store %{d}, %{d}\n", .{ s.ptr.index(), s.val.index() });
             return;
         },
+        .atomic_load => |a| try writer.print("atomic_load %{d} {s} : ", .{ a.ptr.index(), @tagName(a.ordering) }),
+        .atomic_store => |a| {
+            try writer.print("atomic_store %{d}, %{d} {s}\n", .{ a.ptr.index(), a.val.index(), @tagName(a.ordering) });
+            return;
+        },
         // ── Struct ops ──────────────────────────────────────────
         .struct_init => |agg| {
             try writer.writeAll("struct_init [");