feat: #164 Stage B prep — add cancel_observed field to TurnResult

#164 Stage B requires exposing whether cancellation was observed at the
turn-result level. This commit adds the infrastructure field:

Changes:
- TurnResult.cancel_observed: bool = False (query_engine.py)
- _build_timeout_result() accepts cancel_observed parameter (runtime.py)
- Two timeout paths now pass cancel_event.is_set() to signal observation (runtime.py)
- bootstrap command includes cancel_observed in turn JSON (main.py)
- SCHEMAS.md documents Turn Result Fields with cancel_observed contract

Usage:
  When a turn timeout occurs, cancel_observed=true indicates that the
  engine observed the cancellation event being set. This allows callers
  to distinguish:
    - timeout with no cancel → infrastructure/network stall
    - timeout with cancel observed → cooperative cancellation was triggered

Backward compat:
  - Existing TurnResult construction without cancel_observed defaults to False
  - bootstrap JSON output still validates per SCHEMAS.md (new field is always present)

Test results: 182 passing, 3 skipped, zero regression.

Related: #161 (wall-clock timeout), #164 (cancellation observability protocol)
ROADMAP continues #164 with Stage C (test coverage for cancellation + turn envelope).

src/main.py

@@ -332,6 +332,7 @@ def main(argv: list[str] | None = None) -> int:
                     'prompt': session.turn_result.prompt,
                     'output': session.turn_result.output,
                     'stop_reason': session.turn_result.stop_reason,
+                    'cancel_observed': session.turn_result.cancel_observed,
                 },
                 'persisted_session_path': session.persisted_session_path,
             }