check_cards: WRONG's remedy claims to name every copy of a card figure but misses MECHS extra: prose and cross-reference text

[realmarcin]

From the round-5 review of #212 (follows #248).

Where: scripts/fleet/check_cards.py main() closing line, _fleet/README.md's WRONG bullet, and the PR body.

The remedy lists the MECHS records:, card_records and the step 6 pages as "every copy". A card figure also appears in:

• the MECHS extra: prose, for example "625,960 taxa…" and "6,288 deduplicated canonical media…";
• cross-reference text in _fleet/mechs_template.md and _fleet/fleet_fragment.html.

A fixer following the list leaves those behind.

Fix: tell the fixer to grep the tree for the old figure, as step 6 of the update skill does, and fix every occurrence. Name the extra: prose as one of them.