A commit page highlights only the lines it shows: `highlight` under 15 ms for a 100 KB commit #6183

Measured 2026-10-26: /trevato/forge/commit/13cdb24e reports highlight;dur=71.7, which is most of its 104 ms.

Done when:

• [ ] The commit page highlights the hunk lines it draws, not whole files. A file collapsed by default is not highlighted until it is opened.
• [ ] The highlighted output for a drawn line is the same as before: a test compares one hunk.
• [ ] A test with a commit touching 20 files of 800 lines asserts highlight under 15 ms.

Files: the commit page in src/web/pages/, src/web/highlight*.ts, a test. Risk: multi-line constructs (a string or comment opened above a hunk) may colour differently. Accept that and say so in the pull, or seed the tokenizer from the hunk header.