Corpus 100K: Coq — fix extractor (14 → 100K+ from existing CoqGym)

[hyperpolymath]

Part of #12 campaign.

Observation: ECHIDNA has 235 MB of CoqGym corpus at
`external_corpora/CoqGym/` but `stats_coqgym.json` reports only
14 records. This is a ~4-order-of-magnitude shortfall vs the roughly
70 K proof states CoqGym is known to contain.

Action: audit `scripts/extract_coqgym.jl`; likely a regex /
gating bug. Rerun after fix and confirm ≥ 50 K records before
moving on.

Size estimate: CoqGym upstream ≈ 70 K, mathcomp adds ≈ 15 K,
Iris adds ≈ 10 K → realistic target 100 K+ without vendoring
anything new.

[hyperpolymath]

Done in commit 839335f. Rewritten parser yields 85 190 proof states (6 085× prior output), 478 180 tactics, 62 091 premises. ID range 47274–132463.