[AZ-594] Implement core-three harness stubs (fdr_reader, frame_source_replay, imu_replay)

Replaces the NotImplementedError stubs AZ-406 reserved on three runner-
side helpers; these were stranded from any tracker ticket since
AZ-407/408 never came back to fill them. Concrete bodies:

* fdr_reader.iter_records: JSONL parser + wire-envelope validator;
  recursive *.jsonl walk; projects {schema_version, ts, producer_id,
  kind, payload} to runner-side FdrRecord with record_type/monotonic_ms
  renames; yields oldest-first.
* frame_source_replay.replay_video: OpenCV VideoCapture decode + JPEG
  re-encode; auto-detects file vs directory; injectable sleep_fn for
  unit-test pacing.
* imu_replay.ImuReplayer.replay: csv.DictReader parse; degrees->radians
  attitude conversion; tolerates scientific notation; same sleep_fn
  injection pattern.

Adds 34 unit tests (14 + 10 + 10). Full e2e unit suite: 558 passed (+31).
Existing scenario _harness_helpers_implemented probes still return False
because they also depend on sitl_observer / fc_proxy_runtime stubs that
remain pending; scenario probe cleanup is out of AZ-594 scope.

Co-authored-by: Cursor <cursoragent@cursor.com>

_docs/_autodev_state.md

@@ -12,9 +12,9 @@ sub_step:
 retry_count: 0
 cycle: 1
 tracker: jira
-last_completed_batch: 73
+last_completed_batch: 74
 last_cumulative_review: batches_70-72
-current_batch: 74
+current_batch: 75
 current_batch_tasks: ""
 last_step_outcomes:
   step_8: "Code is testable — no changes needed (testability_assessment.md committed; no list-of-changes, no source edits)"