feat(docs): generate the changelog from git and keep it current in maintenance

The page at spaces/agando/downloads/changelog.html was hand-built from VFS notes by the
in-page agent: 11 entries, huge honest holes ("v4.6.59 - v4.6.100 unknown"), and it had
stopped at v4.6.101 while the app was running v4.6.119. A docs page that lags the release
reads as current, which is worse than no page.

- scripts/build-changelog.py derives every entry from git: it walks commits touching
  spaces/newos4, stamps each with the LATEST_VER that was live at that commit, and groups
  them into builds. Kinds map onto the page's own vocabulary (feat->release, fix->fix,
  test->review, docs->docs, chore/style/refactor->project), and a build's chip is its
  dominant kind. Commits whose prefix carries `backticks` render as <code>, escaped first.
- The page is its own template: everything visible comes from the #changelog-data JSON
  island, so regeneration swaps the island and refreshes the two prose blocks that
  described the VFS-only source. No second copy of the CSS or the 296-line renderer.
- Every unshowable thing is stated, not smoothed: commits before the first version stamp
  become one "before v4.6.10" bucket, build numbers that were bumped twice before a commit
  are called out as absent, and the page says entries summarise subjects, not diffs.
- CHANGELOG.md is generated from the same data between markers (app-scoped; the root
  CHANGELOG.md documents the platform's 0.2.x line and is left alone).
- newos4-maintain.py gains check_changelog(): FAIL when the page's build differs from
  LATEST_VER, regenerate under --fix, PASS when current. So the daily 07:15 job keeps the
  docs in step with the build instead of relying on someone remembering.

Verification:
- build: 68 entries, 158 commits, v4.6.10 - v4.6.119, page 63 327 bytes, CHANGELOG.md 15 840.
- live page renders 68 entries with kind chips, search, per-build anchors ("Copy a direct
  link to v4.6.119"), "68 of 68".
- stale detection proved by bumping LATEST_VER to v4.6.120 without rebuilding:
  "FAIL: changelog changelog stale: page says v4.6.119, build is v4.6.120"; --fix then
  "PASS: changelog regenerated: ... v4.6.10 - v4.6.120"; version reverted, page back to 119.
- maintain: PASS node tests 20/20, PASS changelog current: v4.6.119 (68 entries), result OK.
- mobile: verify-mobile.py at 360/390 px, light+dark -> ALL VIEWPORTS OK, 0 overflow.

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