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/e2e/test_download.py

@@ -5,8 +5,6 @@ from pathlib import Path
 
 import pytest
 
-pytestmark = [pytest.mark.unit]
-
 from gps_denied.testing.download import (
     DATASET_REGISTRY,
     DatasetSpec,
@@ -14,6 +12,8 @@ from gps_denied.testing.download import (
     verify_sha256,
 )
 
+pytestmark = [pytest.mark.unit]
+
 
 def test_registry_has_euroc_machine_hall():
     assert "euroc_machine_hall" in DATASET_REGISTRY