atomics A.3a: swap + fence ops + recognizer, emit bails (lock)

swap (atomicrmw xchg) and a standalone fence wired end-to-end except LLVM
emission (both bail loudly; A.3b makes them real).
- RmwKind += xchg; atomic_swap intrinsic + swap method reuse the atomic_rmw op.
- new atomic_fence op (+ AtomicFence) — ordering-only, void; fence($o)/atomic_fence
  intrinsic; recognizer rejects .relaxed (LLVM has no monotonic fence).
- comptime_vm: xchg = store operand/return old; fence = no-op (single-thread).
- examples 1703 (swap) + 1704 (fence) locked to bails; 1187 (relaxed-fence reject).
- 1186 converted to a direct-intrinsic call → stable user-file diagnostic span
  (the lib-forward-site span shifted when atomic.sx grew — fragile-snapshot fix).

Also fixes a latent A.2 comptime-CAS bug found while here: the success/null
has_value write was 'writeWord(addr, SIZE=0, val=1)' — a 0-byte no-op, correct
ONLY because allocBytes zero-inits (REJECTED-PATTERNS 'coincidentally correct').
Now writes the flag explicitly (size=1, val=0). Suite green (721/0).

examples/expected/1186-diagnostics-atomic-cas-ordering.stderr

@@ -1,5 +1,5 @@
 error: atomic compare-exchange failure ordering ('.seq_cst') cannot be stronger than the success ordering ('.relaxed')
-  --> /Users/agra/projects/sx/library/modules/std/atomic.sx:79:188
+  --> examples/1186-diagnostics-atomic-cas-ordering.sx:11:50
    |
-79 |     compare_exchange      :: (self: *Atomic(T), expected: T, desired: T, $success: Ordering, $failure: Ordering) -> ?T { return atomic_cmpxchg(T, @self.value, expected, desired, success, failure); }
-   |                                                                                                                                                                                            ^^^^^^^
+11 |     _ := atomic_cmpxchg(i64, @n, 0, 1, .relaxed, .seq_cst);
+   |                                                  ^^^^^^^^

examples/expected/1187-diagnostics-atomic-fence-relaxed.exit

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

examples/expected/1187-diagnostics-atomic-fence-relaxed.stderr

@@ -0,0 +1,5 @@
+error: fence ordering cannot be .relaxed (use .acquire / .release / .acq_rel / .seq_cst)
+  --> examples/1187-diagnostics-atomic-fence-relaxed.sx:7:18
+   |
+ 7 |     atomic_fence(.relaxed);
+   |                  ^^^^^^^^

examples/expected/1187-diagnostics-atomic-fence-relaxed.stdout

@@ -0,0 +1 @@
+

examples/expected/1703-atomics-swap.exit

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

examples/expected/1703-atomics-swap.stderr

@@ -0,0 +1 @@
+error: atomic swap (xchg) LLVM emission not yet implemented (Stream A, A.3b)

examples/expected/1703-atomics-swap.stdout

@@ -0,0 +1 @@
+

examples/expected/1704-atomics-fence.exit

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

examples/expected/1704-atomics-fence.stderr

@@ -0,0 +1,3 @@
+error: atomic fence LLVM emission not yet implemented (Stream A, A.3b)
+error: atomic fence LLVM emission not yet implemented (Stream A, A.3b)
+error: atomic fence LLVM emission not yet implemented (Stream A, A.3b)

examples/expected/1704-atomics-fence.stdout

@@ -0,0 +1 @@
+