feat(01-03): move ORB + SequentialVO into components/vio/orbslam_backend.py

- Extract SequentialVisualOdometry and ORBVisualOdometry from core/vo.py
  into a dedicated pure-Python OpenCV backend module.
- Module deliberately does NOT import cuvslam — keeps optional-SDK
  isolation from the cuvslam backend (Plan 01-03 Task 1).
- Both classes inherit from the components.vio.protocol.ISequentialVisualOdometry
  Protocol alias (Plan 01-02 surface).
- Barrel-export both classes from components/vio/__init__.py.
- core/vo.py is unchanged in this commit; the shim wires up in Task 4.

src/gps_denied/components/vio/__init__.py

@@ -0,0 +1,29 @@
+"""VIO component (ARCH-01).
+
+Public surface for visual-inertial odometry adapters. Phase-1 split of
+the legacy ``core/vo.py`` monolith into per-backend modules:
+
+  - protocol.py         — VisualOdometry Protocol (alias ISequentialVisualOdometry)
+  - orbslam_backend.py  — pure-Python OpenCV: SequentialVisualOdometry + ORBVisualOdometry
+  - cuvslam_backend.py  — Jetson cuVSLAM SDK bridge: CuVSLAMVisualOdometry + CuVSLAMMonoDepthVisualOdometry
+  - factory.py          — create_vo_backend env-aware DI seed
+  - native/             — placeholder for future cuvslam SDK native glue
+
+The legacy ``gps_denied.core.vo`` import path is preserved as a thin
+re-export shim for one phase; tests still import from there.
+"""
+from gps_denied.components.vio.protocol import (
+    ISequentialVisualOdometry,
+    VisualOdometry,
+)
+from gps_denied.components.vio.orbslam_backend import (
+    ORBVisualOdometry,
+    SequentialVisualOdometry,
+)
+
+__all__ = [
+    "VisualOdometry",
+    "ISequentialVisualOdometry",
+    "ORBVisualOdometry",
+    "SequentialVisualOdometry",
+]