Generate the terms page from TERMS.md, and preserve existing anchors

[ericgriffin]

Summary

Follow-up to #8. Extends the sync to terms/index.html, and fixes an anchor
problem the renderer in #8 would have caused.

The anchor bug, which is the important part

Anchors are a published interface: the site's own cross-links and any external
link point at them. The hand-written pages used short ids (acceptance,
warranties, third-party) that a slug of the full heading does not
reproduce.

Generating the terms page blind would have broken 9 of its 13 anchors, and
#8 had already changed changes to changes-to-this-policy on the privacy
page without anyone noticing.

The renderer now reads the ids off the page before replacing it and reuses the
one an unchanged heading already has. No mapping file to maintain, and a
heading whose text genuinely changed still gets a fresh slug, which is correct
because it is a different section.

Anchor sets diffed against origin/main:

Page	Anchors before	After	Lost
terms	13	13	none
privacy	11	14	cloud-backup only

cloud-backup is a genuine loss rather than a regression: the policy rewrite
restructured that section into "Backup and Sync" and "Photo and Video Upload".
A link to #cloud-backup still lands on the page, just unscrolled.

Terms had drifted too

Less visibly than privacy. TERMS.md says "Terms of Use" throughout where this
page said "Terms of Service". Otherwise the two agree: comparing the body text
against the old page gives 99.4% identity, and every difference is that one
wording change coming from the source.

Workflow

Renders both pages and commits them together, so two renders cannot race each
other pushing to the same branch. The --check job on pull requests now covers
both pages and reports each failure against its own file.

Verification

• Both pages rendered from live fetches of the same URLs the workflow uses, and
  --check clean afterwards
• Rendering twice is byte-identical, so scheduled runs will not produce no-op
  commits
• Both pages parse with balanced tags; section counts match their sources
  (privacy 14 headings / 3 tables / 30 items, terms 13 headings / 11 items)
• Terms body text compared against the old page as described above

One thing to decide, not changed here

The terms body now says "Terms of Use", following the source, while the page's
hand-written title, the nav links and the footer still say "Terms of Service".
Those now disagree. Resolving it is a naming decision rather than a sync one:
either rename the site chrome to match the source, or change TERMS.md.