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+---
+phase: "01"
+plan: "06"
+subsystem: mavlink_io
+tags: [refactor, hexagonal, mavlink, shim]
+dependency_graph:
+  requires: [01-05]
+  provides: [components/mavlink_io]
+  affects: [core/mavlink.py]
+tech_stack:
+  added: []
+  patterns: [shim-re-export, circular-import-avoidance-via-local-imports]
+key_files:
+  created:
+    - src/gps_denied/components/mavlink_io/pymavlink_bridge.py
+    - src/gps_denied/components/mavlink_io/mock_mavlink.py
+  modified:
+    - src/gps_denied/components/mavlink_io/__init__.py
+    - src/gps_denied/core/mavlink.py
+decisions:
+  - "Used local (deferred) imports of MockMAVConnection inside MAVLinkBridge methods to avoid circular import between pymavlink_bridge and mock_mavlink"
+  - "Shim re-exports all seven names including three private helpers (_confidence_to_fix_type, _eskf_to_gps_input, _unix_to_gps_time) per CRITICAL warning"
+metrics:
+  duration: "~5 minutes"
+  completed: "2026-05-11"
+  tasks: 5
+  files: 4
+---
+
+# Phase 01 Plan 06: MAVLink I/O Split Summary
+
+Split `core/mavlink.py` (483 LOC) into component modules under `components/mavlink_io/`, replacing the original with a shim that re-exports all public and private names verbatim.
+
+## LOC Distribution
+
+| File | LOC | Contents |
+|------|-----|----------|
+| `components/mavlink_io/pymavlink_bridge.py` | 455 | MAVLinkBridge class + 3 module-level helpers + pymavlink conditional import |
+| `components/mavlink_io/mock_mavlink.py` | 30 | MockMAVConnection (no-op for dev/CI) |
+| `core/mavlink.py` (shim) | 29 | Re-export shim only |
+| `components/mavlink_io/__init__.py` | 20 | Package surface |
+
+Original: 483 LOC → split across 4 files (514 total including shim and `__init__.py`).
+
+## Test Counts
+
+| Test file | Result |
+|-----------|--------|
+| `tests/test_mavlink.py` | 32 passed |
+| `tests/test_gps_input_encoding.py` | included in the 32 above |
+| Full regression (`tests/`) | 216 passed, 8 skipped |
+
+Regression floor: 216 passed (baseline). Status: MAINTAINED.
+
+## Private Helper Verification
+
+The three underscore names required by `test_mavlink.py` and `test_gps_input_encoding.py` resolve correctly via the shim:
+
+```
+python -c "from gps_denied.core.mavlink import _confidence_to_fix_type, _eskf_to_gps_input, _unix_to_gps_time; print('private helpers ok')"
+# Output: private helpers ok
+```
+
+All three are exported from `core/mavlink.py` → `components/mavlink_io/pymavlink_bridge.py` → callable.
+
+## `_PYMAVLINK_AVAILABLE` on Dev Machine
+
+```
+_PYMAVLINK_AVAILABLE = True
+```
+
+pymavlink is installed on this development machine, so `MAVLinkBridge._open_connection()` will attempt a real connection before falling back to `MockMAVConnection`.
+
+## Deviations from Plan
+
+### Auto-fixed Issues
+
+**1. [Rule 3 - Blocking] Circular import between pymavlink_bridge and mock_mavlink**
+
+- **Found during:** Task 1 (design review before writing)
+- **Issue:** `pymavlink_bridge.py` (MAVLinkBridge) uses `isinstance(self._conn, MockMAVConnection)` checks. If `mock_mavlink.py` were imported at module level in `pymavlink_bridge.py`, and `__init__.py` imports both, a circular import chain would form.
+- **Fix:** Used local (deferred) imports of `MockMAVConnection` inside each method that references it (`_send_gps_input`, `_recv_imu`, `_send_reloc_request`, `_send_telemetry`, `_open_connection`). This matches the pattern used in similar hexagonal refactors in this codebase.
+- **Files modified:** `pymavlink_bridge.py`
+- **Commit:** f965ac7
+
+## Known Stubs
+
+None. All data paths are wired: bridge reads from live ESKFState, writes to real or mock MAVLink connection.
+
+## Threat Flags
+
+None. No new network endpoints or auth paths introduced. The MAVLink UART connection was already present in the original `core/mavlink.py`.
+
+## Self-Check: PASSED
+
+- `src/gps_denied/components/mavlink_io/pymavlink_bridge.py` — FOUND
+- `src/gps_denied/components/mavlink_io/mock_mavlink.py` — FOUND
+- `src/gps_denied/components/mavlink_io/__init__.py` — FOUND (updated)
+- `src/gps_denied/core/mavlink.py` — FOUND (shim, 29 LOC)
+- Commit f965ac7 — FOUND
+- 216 tests passed — CONFIRMED