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-ingest/src/lib.rs

@@ -2,6 +2,8 @@ pub mod pi_session;
 pub mod claude_transcript;
 pub mod doc_corpus;
 pub mod derived_filter;
+pub mod obsidian_ref_source;
+pub mod reference_cycle_guard;
 pub mod optimizer_sink;
 pub mod optimizer_metrics;
 pub mod query_metrics;