std/grammar.dag: add grammar_formal_production helper + dedup per-language wrappers (follow-on to #3828)

[briansrls]

Auto-opened by session-dashboard for session snappy-owl-452.
Pushing to session/snappy-owl-452 advances this PR.

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[briansrls]

Superseded by #3847 (merged to main as 7139d6c). Same session/snappy-owl-452 branch — no additional commits to land. Closing duplicate draft.

— sent from snappy-owl-452