fix(core): catch less common that comparisons

[terminalchai]

AI/agent assistance was used to prepare this patch.

This fixes #3736 by extending the existing ThatThan linter to catch the reported less common that mp3 comparison shape without enabling the broader more/less adjective that pattern that already has known false positives.

The matcher now supports the existing comparative-adjective pattern and a constrained more/less common that <token containing a digit> pattern. I also updated match_to_lint to locate the matched that token dynamically, then added true-positive tests for the reported MP3 comparison and false-positive tests for valid less/more common that ... clauses.

Verification:

• cargo test -p harper-core that_than -- --nocapture
• cargo run -p harper-cli -- lint "This format is way less common that mp3."
• cargo run -p harper-cli -- lint "It is less common that people configure this manually."
• cargo fmt -- harper-core\src\linting\that_than.rs
• git diff --check

[hippietrail]

I'm pretty sure this is going to have false positives along the lines of:

• it's less common that mp3 players break than CD players

[terminalchai]

Good catch, thanks. I narrowed this so the new branch only matches the digit-bearing comparison target at the end of the chunk/sentence, with optional trailing punctuation. That keeps the reported less common that mp3 case covered but avoids cases like it's less common that mp3 players break than CD players.

I added your example as a no-lint regression test and reran:

• cargo test -p harper-core --lib that_than -- --nocapture
• CLI lint checks for the reported typo and the false-positive example
• git diff --check