test(paths): cover lexical normalize and subtree grants

[kirillDevPro]

What & why

crates/moon-util/src/paths.rs had pure path helpers no existing test reached. This adds six unit tests for the branches a caller would notice: lexical .. / . normalization, a .. that climbs out of the path, subtree-grant pruning, and ASCII case-folding of a path component.

Production behaviour is unchanged. The only edit in paths.rs is #[cfg(test)] mod paths_tests;. The existing inline tests in that file were not modified.

Tests and the mutation that proved each one

Each test was run green, the production function was mutated, that test failed for the reason below, the production file was restored (git diff showed only the module declaration), and the test was run green again.

• normalize_lexically_collapses_dot_and_parent_inside_the_path — ParentDir did continue instead of lexical.pop(). Failed assert_eq with left foo\bar and right bar.
• normalize_lexically_rejects_parent_that_escapes_the_root — the root-length check became if false && ..., so a .. that reaches the root was popped instead of rejected. Failed because foo/../../bar returned Ok("bar").
• insert_subtree_prunes_descendants_and_keeps_prefix_siblings — retain kept every grant. Failed assert_eq: left still contained proj/src and proj/tests.
• insert_subtree_ignores_a_path_an_existing_grant_covers — the already-covered early return was skipped (if false &&). Failed assert_eq: left gained proj/src.
• path_within_subtree_matches_the_grant_and_not_a_prefix_sibling — path.starts_with(granted) was inverted. Failed the assertion that proj2 must stay outside a proj grant.
• component_matches_ignore_ascii_case_folds_ascii_case — eq_ignore_ascii_case became eq. Failed the assertion that .ZED matches .zed.

A review with a clean context kept all six. Nothing was dropped. Nothing was left unverified.

Still untested in this file

• PathExt::try_from_bytes
• PathExt::local_to_wsl
• PathExt::try_shell_safe
• path_ends_with
• SanitizedPath::from_arc, new_arc, cast_arc, cast_arc_ref
• RemotePathBuf::from_str, to_proto
• PathWithPosition::map_path
• PathMatcher::is_match_std_path
• compare_rel_paths

How to verify

cargo test -p moon-util --lib paths_tests