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>

src/gps_denied/core/mavlink.py

@@ -4,18 +4,18 @@ CRITICAL: tests/test_mavlink.py and tests/test_gps_input_encoding.py import
 private helpers from this path. Per PATTERNS.md §6.2, the underscore names
 MUST be re-exported here verbatim or 12+ tests break.
 """
+from gps_denied.components.mavlink_io.mock_mavlink import (  # noqa: F401
+    MockMAVConnection,
+)
 from gps_denied.components.mavlink_io.protocol import (  # noqa: F401
     MAVLinkBridgeProtocol,
 )
 from gps_denied.components.mavlink_io.pymavlink_bridge import (  # noqa: F401
-    MAVLinkBridge,
     _PYMAVLINK_AVAILABLE,
-    _unix_to_gps_time,
+    MAVLinkBridge,
     _confidence_to_fix_type,
     _eskf_to_gps_input,
-)
-from gps_denied.components.mavlink_io.mock_mavlink import (  # noqa: F401
-    MockMAVConnection,
+    _unix_to_gps_time,
 )
 
 __all__ = [