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).

examples/1700-atomics-load-store.sx

@@ -1,17 +1,17 @@
-// Atomic($T) load/store (seq_cst), single-thread.
-// Stream A (atomics) A.0 — the first net-new atomic codegen example.
-// Explicit orderings (a.load(.acquire)) arrive in A.0.5; see PLAN-ATOMICS.md.
-// Atomics is an opt-in import (not in the universal prelude) — like `trace`.
+// Atomic($T) load/store with explicit memory orderings, single-thread.
+// Stream A (atomics) A.0 + A.0.5 — the ordering is a comptime `$o: Ordering`
+// param (explicit, Rust-style): a.load(.acquire) emits `load atomic … acquire`.
+// An invalid combination (a.load(.release)) is a compile error (see 1131).
 #import "modules/std.sx";
 #import "modules/std/atomic.sx";
 
 main :: () {
     a := Atomic(i64).init(7);
-    print("init: {}\n", a.load());
+    print("init: {}\n", a.load(.seq_cst));
 
-    a.store(42);
-    print("after store: {}\n", a.load());
+    a.store(42, .release);
+    print("after store: {}\n", a.load(.acquire));
 
-    a.store(a.load() + 1);
-    print("incremented: {}\n", a.load());
+    a.store(a.load(.relaxed) + 1, .seq_cst);
+    print("incremented: {}\n", a.load(.seq_cst));
 }