feat: M3.6 complete (6/6) - reference corpora infrastructure

- M3.6.2: ObsidianRefSource (fetch + chunk from Obsidian API)
- M3.6.4: ReferenceCycleGuard (prevent R re-entry as evidence)
- M3.6.5: QueryLevels (multi-tier filtering, R opt-in)
- M3.6.6-8: Composition gate + enrichment + deduplication
- Tests: 12 assertions validating no system regression

crates/mem-core/src/lib.rs

@@ -2,6 +2,7 @@ pub mod domain;
 pub mod lesson;
 pub mod symptom_projection;
 pub mod query;
+pub mod query_levels;
 pub mod prompt;
 pub mod gate_parser;
 pub mod gated_loop;