atomics A.0c: harden guards (scalar-kind, ordering validity, align bail)

Adversarial review of A.0 found two silent-wrong defects reachable via the public
atomic_load/atomic_store intrinsics (raw LLVM verifier errors, not clean sx
diagnostics) + a latent alignment fallback. All fixed:

- scalar-kind allowlist (call.zig): the size-only T guard admitted same-sized
  aggregates ([8]u8, 8-byte structs) -> invalid 'load atomic [8 x i8]'. Now an
  allowlist switch (integer/float/bool/pointer/enum/vector) rejects loudly.
- per-op ordering validity (call.zig): load cannot release/acq_rel, store cannot
  acquire/acq_rel -> loud diagnostic instead of invalid LLVM.
- val_ty align fallback (ops.zig): the 'else .i64' (align 8) default would
  over-align a sub-8 store -> now bails loudly on a missing val_ty.

Locked by examples 1130 (non-scalar) + 1131 (bad ordering). Suite green (713/0).

examples/1131-diagnostics-atomic-bad-ordering.sx

@@ -0,0 +1,9 @@
+// An atomic load with an ordering LLVM forbids (.release / .acq_rel) is
+// rejected with a clean diagnostic. Stream A (atomics) guard.
+#import "modules/std.sx";
+#import "modules/std/atomic.sx";
+
+main :: () {
+    n : i64 = 0;
+    x := atomic_load(i64, @n, .release);
+}