atomics A.2a: CAS ops + recognizer + methods, emit bails (lock)

compare_exchange/_weak wired end-to-end except LLVM emission (bails loudly;
A.2b makes it real). New IR op atomic_cmpxchg + AtomicCmpxchg{ptr, cmp, new,
val_ty, success_ordering, failure_ordering, weak}; result type = ?T (null =
SUCCESS, failure carries the actual value for retry). print arm; emit dispatch
-> emitAtomicCmpxchg (BAILS). comptime_vm arm does real single-thread CAS (read
actual / compare / store-on-equal / build ?T: success->none, failure->some;
weak == strong at comptime). Recognizer extended (atomic_cmpxchg/_weak, 6 args)
-- CAS restricted to INTEGER T (loud reject); BOTH orderings resolved via
atomicOrderingFromNode; dual-ordering validation (failure may not be
release/acq_rel nor stronger than success, via atomicOrderingRank). Methods
compare_exchange/_weak on Atomic($T) with comptime $success/$failure: Ordering.
examples/1702 locked to the bail; examples/1186 locks a rejected ordering pair.
Suite green (718/0).

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

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

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

@@ -0,0 +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
+   |
+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); }
+   |                                                                                                                                                                                            ^^^^^^^

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

@@ -0,0 +1 @@
+

examples/expected/1702-atomics-cas.exit

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

examples/expected/1702-atomics-cas.stderr

@@ -0,0 +1,3 @@
+error: atomic compare-exchange LLVM emission not yet implemented (Stream A, A.2b)
+error: atomic compare-exchange LLVM emission not yet implemented (Stream A, A.2b)
+error: atomic compare-exchange LLVM emission not yet implemented (Stream A, A.2b)

examples/expected/1702-atomics-cas.stdout

@@ -0,0 +1 @@
+