Serve Learn Concept Map

D7 public concept-map landing: the curated concept taxonomy every lesson/question is tagged against, rendered as a crawlable credibility surface. MUST be declared BEFORE the /learn/{path:path} catch-all (same ordering trap the W2-6 track hub pins). Static data — reads data/learn_concept_taxonomy.json only; no PG, no per-user state, no authed redirect (methodology content serves everyone, like /methodology/*).