docs: add a runnable example to Screen::mask_matching

[cestercian]

What & why

mask_matching explains why masking keeps the grid's width, but nothing compiles an example of it (the README one is rust,ignore). This adds a PTY-free doctest: parse a one-row screen with a clock, mask 12:34:56 with #, and assert both the masked text (Build ######## ok) and that the size did not change. Docs only, no signature changes.

Closes #543

Checklist

• Linked an issue (or explained above why none exists)
• Tests added/updated for the change (the doctest is the change)
• cargo fmt --all and cargo clippy --workspace --all-targets --all-features are clean
• All commits are signed off (git commit -s)
• No AI attribution trailers
• CHANGELOG.md updated under [Unreleased] (user-facing changes only): n/a, doc comment only
• Snapshot changes (if any) were reviewed with cargo insta review, not blind-accepted: n/a

cargo test --doc -p termlens mask_matching and RUSTDOCFLAGS='-D warnings' cargo doc --no-deps pass.