TurnEvent had no trace_hash field, so teaching/discovery._trace_hash
always returned "" via getattr default. Every persisted DiscoveryCandidate
had source_turn_trace="" — provenance gap observed in a real 103-turn
session.
- Add trace_hash: str = "" to TurnEvent
- runtime.finalize_turn_trace_hash back-stamps last TurnEvent and
unstamped tail of _pending_candidates, then re-persists
- CognitiveTurnPipeline.process calls finalize_turn_trace_hash after
compute_trace_hash, before constructing CognitiveTurnResult
Invariants: empty hash is a no-op; back-walk halts at first already-
stamped candidate (no overwrite of prior turns); trace_hash bytes are
unchanged for any given turn.
Validated: tests/test_adr_0153_trace_hash_backstamp.py (6 tests),
core test --suite cognition/smoke/runtime/teaching all green.
Out of scope: OOV candidate trace_hash (same root cause, line-streamed
sink requires different fix); telemetry-sink trace_hash exposure.
