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/expected/1130-diagnostics-atomic-nonscalar.exit

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

examples/expected/1130-diagnostics-atomic-nonscalar.stderr

@@ -0,0 +1,5 @@
+error: atomic ops require a scalar type (integer/float/bool/pointer/enum/vector) of size 1/2/4/8/16 bytes — '[8]u8' is not eligible
+  --> examples/1130-diagnostics-atomic-nonscalar.sx:8:22
+   |
+ 8 |     x := atomic_load([8]u8, @arr, .seq_cst);
+   |                      ^^^^^

examples/expected/1130-diagnostics-atomic-nonscalar.stdout

@@ -0,0 +1 @@
+

examples/expected/1131-diagnostics-atomic-bad-ordering.exit

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

examples/expected/1131-diagnostics-atomic-bad-ordering.stderr

@@ -0,0 +1,5 @@
+error: atomic load ordering cannot be .release or .acq_rel (use .relaxed / .acquire / .seq_cst)
+  --> examples/1131-diagnostics-atomic-bad-ordering.sx:8:31
+   |
+ 8 |     x := atomic_load(i64, @n, .release);
+   |                               ^^^^^^^^

examples/expected/1131-diagnostics-atomic-bad-ordering.stdout

@@ -0,0 +1 @@
+