[AZ-600] Batch 80: refactor sitl_replay_builder to strategy pattern

Replace per-scenario fixture builders with a parameterized strategy
framework so future Derkachi-based scenarios compose existing pieces
instead of duplicating ~200 lines of orchestration per scenario.

New e2e/fixtures/sitl_replay_builder/builder.py:
- VideoSource ABC + StillImagesSource, Mp4PassthroughSource
- TlogSource ABC + SyntheticStationaryTlog, ImuCsvTlog
- FdrProjection ABC + RawFdrPassthrough, OutboundMessagesProjection
- FixtureBuilderConfig + build_fixtures(cfg) orchestrator
- Consolidated MAVLink pack_raw_imu / pack_attitude helpers
- Consolidated run_gps_denied_replay + write_observer_fixture

build_p01_fixtures.py: 423 -> 107 lines (75% reduction).
build_p02_fixtures.py: 292 -> 98 lines (66% reduction).
_common.py: deleted (folded into builder.py).

Tests reorganized:
- test_sitl_replay_builder_builder.py (new, 33 strategy-level tests)
- test_sitl_replay_builder.py (slimmed, 6 FT-P-01 integration)
- test_sitl_replay_builder_p02.py (slimmed, 7 FT-P-02 integration)

README documents the strategy framework + a worked example for
adding FT-P-04 in ~30 lines (no new strategy code required).

Regression gate: 700 passing (was 686; +14 from finer-grained
coverage of new strategy classes and the build_fixtures orchestrator).

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: 79
+last_completed_batch: 80
 last_cumulative_review: batches_76-78
-current_batch: 80
+current_batch: 81
 
 last_step_outcomes:
   step_8: "Code is testable — no changes needed (testability_assessment.md committed; no list-of-changes, no source edits)"