fix(lint): resolve all ruff E402/I001/F821 errors

- Move pytestmark after all imports in 35 test files (E402: not-at-top)
- Add TYPE_CHECKING guard for FlightProcessor in composition.py (F821)
- Sort import blocks in src/ and tests/ (I001 auto-fix via ruff --fix)
- ruff check src/ tests/ now exits 0 with no errors

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_mavlink.py

@@ -14,8 +14,6 @@ import time
 import numpy as np
 import pytest
 
-pytestmark = [pytest.mark.unit]
-
 from gps_denied.core.mavlink import (
     MAVLinkBridge,
     MockMAVConnection,
@@ -27,6 +25,8 @@ from gps_denied.schemas import GPSPoint
 from gps_denied.schemas.eskf import ConfidenceTier, ESKFState
 from gps_denied.schemas.mavlink import GPSInputMessage, RelocalizationRequest
 
+pytestmark = [pytest.mark.unit]
+
 # ---------------------------------------------------------------
 # Helpers
 # ---------------------------------------------------------------