check_is_ascii runs on no CI platform #718

slice::ascii::verify::check_is_ascii is now compiled out on both CI platforms:

• x86_64 + SSE2 (pre-existing): the loop invariant needs modifies clauses for loop invariants, which Kani doesn't support.
• aarch64 + NEON (added in Merge subtree update for toolchain nightly-2026-09-25 #687): is_ascii_neon uses vmaxvq_u8, i.e. simd_reduce_max, which Kani doesn't support (Support simd_reduce_max (reached by core::arch::aarch64::vmaxvq_u8 / <[u8]>::is_ascii) kani#4954).

So the harness currently runs nowhere in CI. Remove each cfg exclusion once its Kani limitation is fixed.