docs: add Ontology Evolution page

Covers the 15 attribute-touching methods + the alignment invariant
enforced by atomic add_attribute / drop_attribute. Documents:

- The four method groups (pure declarations, mechanical migration,
  atomic attribute evolution, opportunistic discovery).
- Why add_attribute is atomic — invariant + commit ordering.
- Failure & retry model (OntologyEvolutionError + idempotent retry
  via chunk markers).
- Type-change pattern (drop_attribute + add_attribute, LLM re-derives).
- Concurrency rule — don't run ingest() during attribute evolution.
- End-to-end example.
- EvolutionResult / OntologyEvolutionError reference tables.
- What's not in the API and why (retype, public backfill_attribute,
  backfill_attribute_semantic).

Wired into mkdocs nav between Graph Schema and Benchmark. Cross-linked
from index.md Next Steps and from the top of graph-schema.md.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>