[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/02_tasks/done/AZ-594_harness_stubs_core_three.md

@@ -0,0 +1,91 @@
+# Harness stubs — core three (fdr_reader, frame_source_replay, imu_replay)
+
+**Task**: AZ-594_harness_stubs_core_three
+**Name**: Implement the three foundational e2e/runner/helpers/ stubs that AZ-406 reserved
+**Description**: Replace `NotImplementedError` stubs in `fdr_reader.iter_records`, `frame_source_replay.replay_video`, and `imu_replay.ImuReplayer.replay` with concrete bodies. Together they unblock the skip-gated scenarios from batches 71-73 once `sitl_observer` lands.
+**Complexity**: 4 points
+**Dependencies**: AZ-406, AZ-407, AZ-408
+**Component**: Blackbox Tests / Test Infrastructure (epic AZ-262)
+**Tracker**: AZ-594
+**Epic**: AZ-262 (E-BBT)
+
+## Problem
+
+AZ-406 committed to the public surface of the runner-side helpers and
+reserved each method body as `NotImplementedError`. AZ-407/408 filled
+the fixture-builder side; the runtime replay/read surfaces were
+deferred without a dedicated ticket. The skip-gated scenarios in
+batches 71-73 (`test_ft_p_07/08/10/11`, `test_ft_n_01..04`) reference
+these stubs in their `_harness_helpers_implemented` probe and skip
+cleanly until the bodies land.
+
+## Outcome
+
+- `fdr_reader.iter_records` reads `*.jsonl` files under an archive
+  root and yields `FdrRecord` envelopes ordered by `monotonic_ms`.
+- `frame_source_replay.replay_video` decodes `.mp4` files or
+  directories of `.jpg`/`.png` frames via OpenCV and emits to the
+  injected `FrameSink` at the requested cadence.
+- `imu_replay.ImuReplayer.replay` parses `data_imu.csv` and drives a
+  `FcInboundEmitter` at the configured rate.
+- Each body has ≥5 unit tests (happy path + ≥2 error paths + ≥1
+  boundary case).
+
+## Scope
+
+### Included
+- The three method bodies + their unit tests.
+- Layout-test entries (already present for the helper files).
+
+### Excluded
+- `sitl_observer.get_observer` + `read_*` surfaces — separate ticket.
+- `fc_proxy` runtime driver — separate ticket.
+- Removing the skip-gates in existing scenarios — happens once
+  `sitl_observer` lands.
+
+## Acceptance Criteria
+
+**AC-1**: `fdr_reader.iter_records` opens every `*.jsonl` file under
+the archive root, parses each line as JSON, and yields `FdrRecord`
+envelopes sorted by `monotonic_ms`. Raises `FileNotFoundError` on
+missing root.
+
+**AC-2**: `frame_source_replay.replay_video` accepts an `.mp4` file
+OR a directory of `.jpg`/`.png` frames; decodes via OpenCV; emits to
+the sink at the cadence (encoded FPS if `realtime`, configured FPS
+otherwise). Returns the count emitted. Raises `FileNotFoundError` on
+missing input.
+
+**AC-3**: `imu_replay.ImuReplayer.replay` parses the CSV at
+`csv_path`, constructs `ImuSample`s with accel/gyro/attitude/baro,
+and calls `emitter.emit(sample)` for each row. Returns the count
+emitted. Tolerates scientific-notation floats.
+
+**AC-4**: Each new body has ≥5 unit tests in
+`e2e/_unit_tests/helpers/` covering the happy path + 2 error paths +
+1 boundary case.
+
+**AC-5**: Full e2e unit-test suite passes (regression gate).
+
+## System Under Test Boundary
+
+None — these are runner-side helpers that synthesize the SUT's
+inputs and parse the SUT's FDR output. They do NOT import any
+`src/gps_denied_onboard` symbol.
+
+## Constraints
+
+- Use `opencv-python-headless` (already pinned in
+  `e2e/runner/requirements.txt`).
+- `imu_replay.replay` must not block on wall-clock at the requested
+  rate when called in tests; a `time.sleep`-based pacing path is
+  acceptable for production use but the test must inject a fake
+  clock / cadence=0 to keep unit tests fast.
+- `fdr_reader.iter_records` uses stdlib `json` (orjson is fine but
+  not required at this stage — keep dependencies minimal).
+
+## Document Dependencies
+
+- `_docs/02_document/tests/blackbox-tests.md` § Test infrastructure
+- `_docs/02_tasks/done/AZ-406_test_infrastructure.md`
+- `_docs/02_tasks/done/AZ-407_fixture_builders_static.md`