test: assert the existing language fixtures

[lvolland]

examples/languages/test.* is already the test suite — every fix since #53 adds a line to it. this doesn't add a corpus, it records what the tokenizer produces for yours.

paste this:

git checkout e2e036b~1 -- src/languages/js.js && npm test

today nothing happens. after: 3 red — test.js, test.md, test.ts. md and ts pull js in with a spread, so #70 broke them too. not visible by eye.

reverting each fix in turn, 6/6 go red on the right language: #53 py, #54 json, #56 md, #58 py, #66 ini, #70 js+md+ts.

no dependency, node --test is built in. nothing lands in src/ or dist/; the tarball stays at 263 files, +43 bytes for the "test" line. 0.2s.

notes

• UPDATE=1 refreshes snapshots, but the "tokens must join back into the source" assert runs first — a refresh can't bless dropped code. with a token-dropping mutation it leaves 35/36 red and writes nothing.
• snapshots freeze current behaviour, warts included — early UPDATE=1 diffs will show improvements that look like regressions.
• .gitattributes pins fixtures and snapshots to -text — without it they check out CRLF on windows and the suite fails 36/36.
• test.yml runs on prs and on push to main, since all six fixes were direct pushes. informational only: release.yml doesn't depend on it.
• tokenisation only, not detect.js, terminal.js or the html output.
• a process change, and nothing says you want one — close it if you'd rather not carry a tests/ dir, no hard feelings 🙂