[AZ-599] Batch 79: FT-P-02 Derkachi builder + _common.py extraction

- Add build_p02_fixtures.py: IMU CSV → tlog conversion (RAW_IMU +
  ATTITUDE pairs, centidegrees→radians yaw) and orchestrator that
  runs gps-denied replay against Derkachi MP4 + generated tlog,
  verifying ≥1 record_type="estimate" in the FDR archive.
- Extract run_gps_denied_replay + FDR-parent-dir helpers into
  sitl_replay_builder/_common.py; refactor build_p01_fixtures.py
  to import from _common (b78 tests preserved).
- Add 20 unit tests under e2e/_unit_tests/fixtures/test_sitl_
  replay_builder_p02.py covering AC-1..AC-5; total unit suite
  686/686 passing (regression gate AC-6).
- README updated to document FT-P-01 + FT-P-02 builders.
- Advance autodev state: last_completed_batch=79, current_batch=80;
  prune verbose detail blob.

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

_docs/03_implementation/batch_79_report.md

@@ -0,0 +1,88 @@
+# Batch 79 Report — FT-P-02 Derkachi fixture builder
+
+**Batch**: 79
+**Date**: 2026-05-17
+**Context**: Test implementation (greenfield Step 10 — Implement Tests)
+**Tasks**: AZ-599 (3 cp) — 1 task (FT-P-02 Derkachi builder + b78 helper extraction)
+**Cycle**: 1
+**Verdict**: COMPLETE — PASS (self-reviewed; see `reviews/batch_79_review.md`)
+
+## Summary
+
+Replicates the b78 vertical-slice pattern for FT-P-02 (Derkachi
+continuous-flight drift). Different from b78 in three ways:
+
+1. **Video is already MP4** — no still-image encoding step.
+2. **IMU is real recorded telemetry** — needs CSV → tlog conversion
+   with real motion data (vs. b78's synthetic stationary tlog).
+3. **Output is the SUT's natural FDR archive directory** — FT-P-02
+   reads it via `fdr_reader.iter_records`, not via the per-call
+   `outbound_messages_*.json` schema b78 ships.
+
+Also extracted `run_gps_denied_replay` + `write_observer_fixture`
+into a shared `_common.py` so both b78 and b79 reference one
+canonical implementation. Future per-scenario builders (FT-P-04
+also uses Derkachi inputs, FT-P-05 needs its own, etc.) will reuse
+the same helpers.
+
+### AZ-599 — FT-P-02 Derkachi builder (3 cp)
+
+* **`e2e/fixtures/sitl_replay_builder/_common.py`** (new): shared
+  `run_gps_denied_replay`, `write_observer_fixture`, `DEFAULT_CLI_BIN`.
+* **`e2e/fixtures/sitl_replay_builder/build_p01_fixtures.py`**
+  (edited): re-imports the moved helpers from `_common.py`; b78
+  tests still pass unchanged.
+* **`e2e/fixtures/sitl_replay_builder/build_p02_fixtures.py`** (new):
+  * `P02BuilderConfig` frozen dataclass.
+  * `convert_imu_csv_to_tlog(csv_path, output_tlog)` — parses
+    `data_imu.csv` rows, packs RAW_IMU + ATTITUDE pairs. Yaw from
+    `GLOBAL_POSITION_INT.hdg` (centidegrees → radians);
+    roll/pitch = 0 (fixed-wing cruise approximation).
+  * `verify_fdr_has_estimates(fdr_path)` — counts
+    `record_type="estimate"` records; raises if zero.
+  * `build_p02_fixtures(cfg, ...)` — orchestrator.
+  * `_main(argv=None)` — argparse CLI.
+* **`e2e/fixtures/sitl_replay_builder/README.md`** (edited): two-
+  builder structure with scenario coverage table; FT-P-01 + FT-P-02
+  per-builder sections with strategy + usage + limitations.
+* **`e2e/_unit_tests/fixtures/test_sitl_replay_builder_p02.py`**
+  (new): +20 tests.
+* **`e2e/_unit_tests/test_directory_layout.py`** (edited): registers
+  the two new modules (`_common.py` + `build_p02_fixtures.py`).
+
+## Out of scope (deferred)
+
+* **Live capture EXECUTION** — same as b78, a manual operator step.
+* **FT-P-04** (Derkachi F2F registration) — also uses Derkachi
+  inputs and could reuse `build_p02_fixtures`. Deferred to a
+  follow-up so this batch ships at 3 cp.
+* **Other scenarios** (FT-P-03 / 05 / 07 / 08 / 10 / 11, FT-N-01..04).
+* **iNav adapter** for FT-P-02.
+* **True SCALED_IMU2 → RAW_IMU unit conversion** — pass-through;
+  re-visit if the SUT's tlog parser rejects.
+* **Aggressive-manoeuvre ATTITUDE synthesis** (roll/pitch ≠ 0) —
+  re-visit if FT-P-04 or other scenarios show fusion drift.
+
+## Test Results
+
+* New unit tests: **20**.
+* Full `e2e/_unit_tests` suite: **686 passed in 123 s** (previous:
+  664 → +22 net = +20 new tests + +2 directory-layout entries).
+* No new linter errors.
+* b78 builder tests (24) still pass after `_common.py` extraction.
+
+## State
+
+* Spec moved: `_docs/02_tasks/todo/AZ-599_ft_p_02_derkachi_builder.md`
+  → `_docs/02_tasks/done/`.
+* `_docs/_autodev_state.md` advanced to `last_completed_batch: 79`.
+* No K=3 cumulative review this batch (next due after b81; the
+  b76-b78 cumulative just shipped one batch ago).
+
+## Incidents
+
+* **Disk-full during regression gate**: the first regression-gate
+  attempt failed with 212 conftest setup errors (all
+  `FileNotFoundError: /var/folders/.../pytest-of-...`). Surfaced
+  to the user with options; user freed space (37 Gi avail after);
+  re-ran the gate and it passed cleanly. Not a code issue.