feat: add medical extensions (MedicalRoleGraph, symbolic embeddings, SNOMED/UMLS extractors)

Adds medical domain support behind `medical` feature flags across three crates:

**terraphim_types**: MedicalNodeType (27 variants), MedicalEdgeType (65 variants)
**terraphim_rolegraph**: MedicalRoleGraph with IS-A hierarchy, adjacency index, symbolic embeddings (Jaccard + path distance)
**terraphim_automata**: EntityExtractor (SNOMED), UmlsExtractor, ShardedUmlsExtractor with daachorse sharded automaton, bincode+zstd artifact serialization

Key fixes:
- magic_unpair f32->f64 for SNOMED IDs (100M-900M range)
- Overlap detection: start < m.span.1 && end > m.span.0
- Multi-CUI term preservation in ShardedUmlsExtractor
- Adjacency index for O(degree) edge lookups
- Clippy: derive(Default) and is_some_and

78 rolegraph tests + 41 automata tests, all passing.
E2E verified in medgemma-competition (49/49 checks pass).

Closes #549

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: AlexMikhalev

.beads/issues.jsonl

@@ -60,3 +60,9 @@ keys/
 crates/terraphim_settings/test_settings/settings.toml
 # Nightshift plan artifacts (keep out of version control)
 .nightshift-plan
+
+# Build artifacts
+a.out
+
+# Cache
+.cachebro/