fix(docs): repair a broken catalog table and four stale homepage counts

[burinc]

Two defects, both mine, both visible on the published site. Both now gated,
because neither was catchable before.

The shaders table rendered as |---|---|---| and nothing else

The basic-lighting row sat ABOVE its table header, so the whole shaders
table collapsed on the site.

My insertion script sorted rows by the text between the first pair of
backticks. The header line | preview | bb name | shows | also contains
backticks, yielding "bb". Since basic-lighting sorts before bb, the
header was chosen as the insert anchor and the row went in front of it. Only
that one row was low enough in the alphabet to hit it, which is why it looked
fine everywhere else.

home.html still said 175 in four places

The call-to-action button, an architecture diagram label, a section heading
and a shell comment. I corrected the headline and per-group counts in the
previous PR and missed these, because I piped the audit grep through
head -20 and it cut the output at exactly the line that would have shown
them. The file contains five 175s, not one.

Gates

Neither defect was reachable by the existing checks: every row was present,
every image resolved, and nothing gated home.html at all.

• The catalog gate now requires each group table to start with its header, and
  each ## group (N) heading to match the rows beneath it.
• The homepage gate now checks the example count wherever it appears, and
  every per-group figure, against the registry.

Each of the five new checks was verified by planting the exact error it is
meant to catch and confirming it fires. That is worth stating because a gate
that cannot fail is worse than no gate: it reads as coverage.