fix: fingerprint the fiber census page as SurrealDB stores it

[RobertSigmundsson]

Summary

• Merge no longer stops with "fiber census staged page is invalid" when a fiber has an empty field.
• The census page is encoded without None-valued keys (recursively) before it is fingerprinted and stored, i.e. exactly as SurrealDB stores it.
• Adds a live-SurrealDB regression test and a CHANGELOG entry.

Why

After upgrading to 3.12.0 the first consolidation run stopped at merge every time ("fiber census staged page is invalid"), and because a failed progress record blocks the other strategies, enrich, mature and semantic_link came back blocked. The census fingerprints the encoded page with its None fields, but the Python SDK (2.0.0, within the pinned surrealdb>=2.0.0,<3.0.0) sends None as NONE and SurrealDB drops NONE-valued keys from the stored record, nested ones inside metadata included. The page read back no longer hashes to page_fingerprint, so validate_page rejects it. Almost every fiber has an empty summary or last_ghost_shown_at, so merge never gets past the first page. Encoding the page the way it will come back fixes the mismatch; decode_fiber already defaults every missing key, so nothing else changes.

Test plan

• pytest tests/ -m "not stress" -n auto passes locally.
• ruff check src/ tests/ clean.
• mypy src/ --ignore-missing-imports clean.
• tests/integration/test_fiber_census_none_fields_db.py (live SurrealDB 3.2.3, SMEM_TEST_SURREALDB_URL): three fibers with summary=None and a None metadata value, then a real merge run - fails on v3.12.0 with the message above, passes with the fix.
• Full-size brain restored from a dump: merge, enrich, mature and semantic_link all complete (semantic_link resumed twice across the 600 s budget).

Note: a run that already failed on 3.12.0 keeps pages fingerprinted the old way and cannot resume even after the fix - its consolidation_progress row and consolidation_fiber_census rows have to be deleted. Maybe worth a line in the release notes, or a CLI way to abandon a failed run.

Verified by

@RobertSigmundsson.