Task: implement the anti-corruption layer (we3_id mapping + CI rule)

[espi]

Not blocked on anything (unless #2 is challenged). Scope from 02-computer-science-data.md, Q2.2:

• Stable we3_ concept ids; mapping table we3_id ↔ (taxonomyVersion, mt_id)
• Ingest pinned by manifest SHA-256 (snapshot already vendored at vendor/os-taxonomy/, upstream commit 96a7933)
• Overlay tables referencing we3_id (corrections, difficulty, new topics)
• CI check: no mt_ id as a foreign key in learner-data tables
• Split/merge review queue for future upstream releases

Estimated ~a day now; a user-data migration with legal review if retrofitted after launch.