docs: update changelog for v1.9.0 Dynamic Ontology release

- Add v1.9.0 entry with Dynamic Ontology features
- Include crates.io reference
- Link to GitHub release

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

Author: Terraphim CI

docs/src/changelog.md

@@ -4,6 +4,26 @@ This is a chronological changelog for Terraphim AI.
 
 ## 2026
 
+### v1.9.0
+
+Release date: 2026-02-20
+
+Highlights:
+- **Dynamic Ontology** - Schema-first knowledge graph construction with coverage governance signals
+- **GroundingMetadata** - Canonical URIs for normalized entities
+- **CoverageSignal** - Quality governance signals for extraction quality
+- **SchemaSignal** - Entity extraction with confidence scores
+- **HgncNormalizer** - Gene normalization (EGFR, TP53, KRAS, etc.)
+- **Feature Gates** - `ontology` (default), `medical`, `hgnc` for flexible dependencies
+
+Published to crates.io:
+```toml
+terraphim_types = "1.6.0"
+```
+
+Reference:
+- https://github.com/terraphim/terraphim-ai/releases/tag/v1.9.0
+
 ### v1.2.3
 
 All binaries are available at: