Unsafe Rust skill: Be more strict about exhaustive Rust version enumeration

Currently, the unsafe Rust skill permits exhaustive enumeration of non-semver-stable guarantees – for example, "while it's not guaranteed, this API behaves like this on versions 1.56 and 1.57, and this code is explicitly gated and only compiles on those two versions".

We should be *slightly* more precise about this: If this sort of guard isn't a *list* of versions but rather a version *range* (as zerocopy uses), then this is technically unsound because there's nothing stopping Rust from later releasing a point release (e.g., 1.56.1) which breaks the assumption.

Locate the unsafe Rust skill guidance and read its current wording and examples about exhaustive Rust version enumeration. Clarify that explicit version lists differ from version ranges, using the 1.56.1 point-release concern as the acceptance case; the issue names no file or test, so verify the updated guidance through the repository's relevant documentation checks.

Written by the indexing model from the issue text.