[AZ-698] Multi-flight tlog handling: segment first, pick last flight

Real derkachi.tlog covers 3 takeoffs at the same field but the
uploaded video covers only the last. Original NCC argmax + AZ-405
head-takeoff fallback both biased toward flight 1, violating the
spec's "the last chunk in tlog is relevant" framing.

Patch: pre-NCC flight segmenter partitions the IMU energy stream
into distinct flights (threshold + gap walk); find_aligned_window
restricts NCC search to the last segment; low-confidence fallback
uses that segment's start instead of head-takeoff detection.
AlignedWindow gains flight_count_detected + selected_flight_index
for FDR-visible audit.

7 new unit tests (segmenter shapes + end-to-end multi-flight
pipeline + segmented fallback path). 19 AZ-698 tests pass, 113
in the regression slice. Zero new mypy --strict errors.

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

_docs/03_implementation/batch_99_cycle2_report.md

@@ -98,6 +98,33 @@ opt-in via constructor kwarg so the `--skip-auto-sync` path is untouched.
 The normalised-cross-correlation switch is documented in the spec's
 "Implementation Notes" appendix as the algorithmic decision of record.
 
+## Follow-up commit: multi-flight handling
+
+User-reported gap during the AZ-698 "In Testing" phase: real
+`derkachi.tlog` contains **three takeoffs**; the video covers only
+the last. The original AZ-698 happy path (`np.argmax`) and fallback
+(`detect_tlog_takeoff` on head) were both biased toward flight 1.
+
+Patched in a follow-up commit on top of batch 99:
+
+- New `_segment_flights_from_imu_energy` helper partitions the IMU
+  energy stream into distinct flights using a motion-threshold +
+  gap-tolerance walk.
+- `find_aligned_window` now restricts NCC search to the **last**
+  detected segment.
+- Low-confidence fallback uses the last segment's start instead of
+  re-running head-takeoff detection on the whole tlog.
+- `AlignedWindow` gains `flight_count_detected` + `selected_flight_index`
+  for observability; both are surfaced in the `replay.auto_trim.resolved` /
+  `…fallback_to_takeoff` log records.
+- New unit tests: segmenter happy paths (1-flight, 3-flight),
+  ground-blip rejection, cruise-lull preservation; integration test
+  proving `find_aligned_window` on a 3-flight tlog picks flight 3.
+
+Test totals after follow-up: **113 passed, 2 skipped, 0 failed.**
+Zero new mypy --strict errors (12 errors in scope, all pre-existing
+and unchanged).
+
 ## Next batch
 
 Batch 100 — **AZ-699** (real-flight validation runner). Depends on