Versioned additive-optional migration (L10 scoping step-2 ruling): a checkpoint
schema bump is a recorded lineage transition, not death-and-rebirth.
- engine_state.load_manifest() now REFUSES (IncompatibleEngineStateError) a
checkpoint whose schema_version > this build's _SCHEMA_VERSION, and tolerates
<= current (older/equal read any missing newer fields via additive-optional
defaults). Never silently mis-loads newer state.
- chat.runtime._load_engine_state() loads the manifest FIRST so the version
refusal gates before any recognizers/candidates are read.
- DerivedRecognizer.from_json documents the additive-optional convention
(new fields .get-defaulted + omitted-when-default), mirroring DiscoveryCandidate.
Tests (TDD): refuses newer schema_version; tolerates older. Prerequisite for the
L10 continuity spike's P2 byte-identity gate (it may now assume a fixed schema
within a run, with version bumps handled explicitly).
0b19e08306