Add `runner/helpers/replay_mode.py` (NullFrameSink, NullFcInboundEmitter,
default_frame_period_ms, load_replay_json, resolve_replay_subdir,
imu_replay_noop) and rewire all 13 scenarios off their local
`_resolve_*` / `_drive_*` / `_push_*` NotImplementedError stubs.
Closes the offline FDR-replay execution path. `grep raise
NotImplementedError` under `e2e/tests/` now returns zero matches. +17
unit tests (626 total, up from 608). Unit-test behaviour unchanged
(scenarios still skip via b75 sitl_replay_ready gate when
E2E_SITL_REPLAY_DIR is unset).
Co-authored-by: Cursor <cursoragent@cursor.com>
Implement all 11 `sitl_observer` public surfaces as an offline
FDR-replay strategy (reads JSON fixtures under `${E2E_SITL_REPLAY_DIR}`
instead of live pymavlink/yamspy). Replace 12 per-scenario
`_harness_helpers_implemented` probes with one shared session-scoped
`sitl_replay_ready` fixture in `e2e/tests/conftest.py`.
Net: -636 LoC of duplicated scenario gating, +17 LoC shared fixture,
+38 new unit tests (596 total, up from 558). Includes K=3 cumulative
review for batches 73-75 (PASS).
Co-authored-by: Cursor <cursoragent@cursor.com>