fix(search): share under_dir with query cwd filter

[Drix10]

Issue: #72

What changed and why?

query --cwd (#51) filtered with its own naive path_under: plain
canonicalize plus byte starts_with. On Windows that misses verbatim
\\?\-prefixed cwds and case variants, the exact bug #67 fixed for
list --cwd.

The under_dir core (verbatim-strip, case-fold compare, lenient
canonicalize) moves from the CLI to new src/paths.rs, and both filters
route through it. Its unit tests move along; query gains a regression
test that failed before the fix (0 hits) and passes after.

Checks: cargo test -p txcript (65 lib + 225 integration) and
cargo test -p txcript-cli --lib (103) all pass; fmt --check and
clippy --all-targets clean on both crates. Also verified through the
built binary: list and query with a differently-cased --cwd now agree.

[Drix10]

Second commit just extends coverage: the moved tests were all Windows-gated, so Linux CI exercised none of the shared logic. Added a platform-neutral test (vanished child matches, same-prefix sibling doesn't) plus verified the wasm32 check passes with the code now living in the lib crate. No behavior change.