v0.6.1 — Topic discipline: repeated facts converge onto one topic

• Topic canonicalization: repeated facts now converge onto one topic instead of sprouting lexical variants; candidates snap to an existing topic at high slug similarity, with a hard veto so slugs carrying different dates are never treated as the same topic.
• The extraction prompt shows the current active-topic list so models reuse canonical topic names instead of inventing near-duplicates.
• Supersede/refine proposals naming a shown, active, type-equal, topic-equivalent memory resolve to a refine of that memory instead of a duplicate-plus-supersede pair.
• A one-time, idempotent startup backfill clusters and renames accumulated topic variants inside a single transaction, keeping the search index, queued jobs, and consolidation checkpoints consistent; no manual migration or configuration needed, and later boots are zero-write no-ops.
• The benchmarks corpus gained a topic-discipline scenario locking the behavior into the evaluation gate. (#54)