Mark covmap functions as unsafe

[lewismosciski]

Description

Fixes #3417

This PR addresses the soundness bug identified in issue #3417, where the safe functions covmap_is_interesting_simd and covmap_is_interesting_naive could cause Undefined Behavior.

Problem:
These functions use get_unchecked based on an assumption that hist.len() >= map.len(). If this invariant is violated by a caller, it results in an out-of-bounds memory access, which is UB in a safe function.

Solution:
This patch resolves the soundness issue by:

1. Marking both functions as unsafe.
2. Adding a # Safety section to their documentation, clearly stating the required invariant for callers.
3. Updating all internal call sites to use unsafe blocks with comments justifying their safety.

Checklist

• I have run ./scripts/precommit.sh and addressed all comments

[tokatoka]

hey could you run scripts/fmt_all.sh to format your changes