feat(02-03): apply module-level pytestmark to 37 test files

- Add pytestmark = [pytest.mark.<category>] to all 23 root test files and 14 e2e test files
- Marker distribution: 22 unit, 7 integration, 1 blackbox, 1 sitl, 5 e2e + 2 e2e integration
- Add import pytest to test_models.py, test_download.py, test_synthetic_adapter.py (were missing)
- Convert test_sitl_integration.py's bare pytestmark to list form preserving skipif guard
- Union of all 5 markers = 298/298 = 100% coverage; 216 tests pass with --strict-markers

tests/e2e/test_euroc_adapter.py

@@ -4,6 +4,8 @@ from pathlib import Path
 
 import pytest
 
+pytestmark = [pytest.mark.unit]
+
 from gps_denied.testing.datasets.base import (
     DatasetNotAvailableError,
     PlatformClass,