atomics A.1a: RMW ops + recognizer + methods, emit bails (lock)

fetch_add/sub/and/or/xor/min/max wired end-to-end except LLVM emission (bails
loudly; A.1b makes it real). New IR op atomic_rmw + RmwKind (no nand) +
AtomicRmw{ptr, operand, val_ty, ordering, kind}. print arm; comptime_vm arm
implements real single-thread RMW (load/compute/store/return-old, signed|unsigned
min/max from val_ty). Recognizer extended (rmwKindFromName) — RMW restricted to
integer T (float fadd / pointer RMW out of scope, rejected loudly); all orderings
valid for RMW. Methods fetch_* on Atomic($T) with comptime $o: Ordering.
examples/1701 locked to the bail. Suite green (716/0).

examples/expected/1701-atomics-rmw.exit

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

examples/expected/1701-atomics-rmw.stderr

@@ -0,0 +1,7 @@
+error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
+error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
+error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
+error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
+error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
+error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)
+error: atomic rmw LLVM emission not yet implemented (Stream A, A.1b)

examples/expected/1701-atomics-rmw.stdout

@@ -0,0 +1 @@
+