chore: improve bytes maintenance path

[vip892766gma]

Summary:

• Audit the BytesMut/Bytes unsafe blocks for missing or imprecise SAFETY comments and add a small targeted test (or doc-test) covering a boundary condition such as split_to/split_off at index 0 and at len(), and verify capacity/refcount invariants — a small code-level patch (test + a SAFETY comment clarification) with no API change.
• Keep the change narrow so it is straightforward to review.

Notes:

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

[seanmonstar]

Thanks!