feat: add content-addressed provider snapshots

[bionicbutterfly13]

Summary

• adds Track 026 for content-addressed provider snapshots
• keeps full provider snapshots as the default compatibility mode
• adds strategy object digests, stable population root hashes, embedded-object snapshots, and manifest-only restore with external object stores
• exposes InMemoryProvider.snapshot(mode="content_addressed", include_objects=...)
• updates status docs and changelog for the 1194-test baseline

Verification

• .venv/bin/pytest tests/unit/envelope/test_snapshot.py tests/contract/test_provider_contract.py -q -> 63 passed
• .venv/bin/pytest -q -> 1194 passed
• .venv/bin/ruff check src tests reference_service/src -> clean
• uv build -> wheel and sdist built successfully