atomics A.0.5: full ordering surface (comptime $o: Ordering)

Migrate Atomic methods from seq_cst-only to the explicit ordering surface now
that comptime value params work on generic-struct methods (workers 3c4305f /
d7a6857 / d95ba0a):

- atomic.sx: load/store take a comptime $o: Ordering (explicit, Rust-style; no
  default, matching design 4.6). a.load(.acquire) -> 'load atomic .. acquire'.
- call.zig: atomicOrderingFromNode resolves a comptime-bound ordering identifier
  via comptimeIntNamed (+ atomicOrderingFromTag); documents the sx-Ordering <->
  IR-AtomicOrdering declaration-order invariant. The per-op validity guard fires
  through the method path (a.load(.release) is a compile error).
- 1700 migrated to explicit orderings (output unchanged 7/42/43).

Suite green (715/0).

current/CHECKPOINT-ATOMICS.md

@@ -59,6 +59,22 @@ Full atomic load/store plumbing with LLVM emission deliberately bailing loudly;
 `.ir`; add `emit_llvm.test.zig` unit. Then adversarial review, then the comptime-enum worker
 + A.0.5 migration to the full ordering surface.
 
+## A.0.5 — full ordering surface (DONE)
+`Atomic($T).load($o: Ordering)` / `store(v, $o)` — ordering is a COMPTIME value param,
+explicit (Rust-style, no default; design §4.6). `a.load(.acquire)` emits `load atomic …
+acquire`; `a.store(v, .release)` emits `store atomic … release`; `a.load(.release)` is a
+compile error (per-op validity guard fires through the method path). Recognizer
+`atomicOrderingFromNode` now resolves a comptime-bound ordering identifier via
+`comptimeIntNamed` (+ `atomicOrderingFromTag`, with the sx-Ordering ↔ IR-AtomicOrdering
+declaration-order invariant documented). 1700 migrated to explicit orderings (output
+unchanged 7/42/43). Suite green (715/0).
+
+**Unblocked by three comptime-value-param commits (workers):** enum (3c4305f), tagged_union
+(d7a6857), generic-struct methods (d95ba0a). NOTE: default VALUES for comptime params on
+generic-struct methods are NOT bound (orthogonal gap — free-fn defaults work); atomics
+sidesteps it cleanly by requiring explicit ordering (matches the design). Candidate
+follow-up, not an atomics blocker.
+
 ## Known issues / capability gaps
 - **Comptime-constant ordering propagation MISSING (blocks the full surface).** A runtime
   `Ordering` method param can't reach LLVM (orderings are instruction attributes, not
@@ -98,3 +114,7 @@ Full atomic load/store plumbing with LLVM emission deliberately bailing loudly;
 - **A.0c** — guard hardening from the adversarial review: scalar-kind allowlist + per-op
   ordering validity (call.zig), val_ty align bail (ops.zig), + diagnostic examples
   1130/1131. Suite green (713/0). (comptime enum value params landed via worker 3c4305f.)
+- **A.0.5** — full ordering surface: `Atomic($T).load/store($o: Ordering)` comptime ordering
+  (explicit). Recognizer resolves comptime-bound ordering via `comptimeIntNamed`. 1700
+  migrated to explicit orderings (acquire/release/relaxed/seq_cst). Unblocked by
+  comptime-value-param workers (3c4305f/d7a6857/d95ba0a). Suite green (715/0).