atomics: close out Stream A (feature-complete)

Final whole-stream adversarial review came back CLEAN (no CRITICAL/MEDIUM/LOW).
Close the one informational gap it noted: extend examples/1703 with a #run
comptime swap so swap's comptime VM arm is locked (742, matches runtime) — every
op now has comptime↔runtime corpus coverage.

Docs: PLAN-ATOMICS.md status banner (COMPLETE); PLAN-POST-METATYPE.md Stream A
marked done (unblocks B2-channels + C-parallel); readme.md gains a user-facing
Atomics section. Suite green (721/0).

examples/1703-atomics-swap.sx

@@ -1,11 +1,16 @@
 // Atomic($T).swap — atomic exchange (LLVM atomicrmw xchg): store the new value,
 // return the OLD one. Stream A (atomics) A.3. Single-thread.
+// Covers swap at BOTH comptime (#run) and runtime — they must agree.
 #import "modules/std.sx";
 #import "modules/std/atomic.sx";
 
+c_swap :: () -> i64 { a := Atomic(i64).init(7); old := a.swap(42, .seq_cst); return old * 100 + a.load(.seq_cst); }
+G_SWAP :: #run c_swap();   // 742 (old 7, now 42)
+
 main :: () {
     a := Atomic(i64).init(7);
     old := a.swap(42, .acq_rel);
     print("swap old: {}\n", old);              // 7
     print("swap now: {}\n", a.load(.acquire)); // 42
+    print("comptime swap: {}\n", G_SWAP);      // 742 (matches runtime)
 }

examples/expected/1703-atomics-swap.stdout

@@ -1,2 +1,3 @@
 swap old: 7
 swap now: 42
+comptime swap: 742