chore: improve memchr maintenance path

[vip892766gma]

Summary:

• Add additional edge-case unit tests in the existing #[cfg(test)] modules covering empty-needle, single-byte haystack, and boundary alignment cases for the generic (all-arch) memchr/packedpair fallback paths, plus a debug_assert for needle length invariants at one public entry point where the invariant is currently only implicit. Pure additive tests and one narrow assertion — no behavior change.
• Keep the change narrow so it is straightforward to review.

Notes:

• I kept this scoped to the relevant implementation and tests.