Address Codex measurement-core review

- Calibration: still derived in angle space via the 180-degree flip, but now
  APPLIED in vector space as a reference-orientation rotation (Rodrigues
  alignment for Surface, Z-rotation for Edge), exact away from zero; tests
  at 30 degrees, cross-axis, and edge-polarity cases.
- Angle relative zero: stores the gravity direction vector; relative reading
  is the angle between directions, so cross-axis movement is honest.
- Edge mode: precise geometry documented (either long edge down, gravity
  along +/-X), placement-validity guard so e.g. Edge mode never locks on a
  phone lying flat; UI shows repositioning hints.
- Lock/readout coherence: locked label states the tolerance (exit threshold)
  so the rounded readout can never contradict it; pipeline acceptance tests
  pin the invariant.
- Sensor-vector contract: documented and pinned by SensorContractTest. The
  suggested negation of gravity/accelerometer fallbacks is NOT applied: per
  Android SensorEvent docs, a stationary flat device reads +9.81 on Z (the
  gravity reaction), matching the rotation-vector path as-is. The contract
  tests prove all paths agree.

38 tests passing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

README.md

@@ -23,12 +23,18 @@ Implemented and tested:
 
 - `core/sensors` — sensor selection (game rotation vector → gravity → low-passed
   accelerometer), device-frame gravity stream, and the pure measurement math:
-  orientation mapping, two-sample per-mode calibration, EMA smoothing,
-  lock hysteresis/dwell/debounce, display deadband. All unit-tested.
+  orientation mapping, two-sample per-mode calibration (derived in angle space,
+  **applied in vector space** so it stays correct away from zero), EMA smoothing,
+  lock hysteresis/dwell/debounce, display deadband, placement validity, and the
+  sensor-vector contract (all sources emit device-frame world-up; flat = +9.81 on Z,
+  pinned by `SensorContractTest`). All unit-tested.
 - `core/settings` — DataStore preferences and per-mode calibration persistence.
 - `core/billing` — entitlement interface with a stub (free-tier) implementation.
 - `feature/level`, `feature/angle` — live numeric scaffolds wired to the real
-  sensor pipeline, with manual Surface|Edge selection, lock state, hold-to-zero.
+  sensor pipeline, with manual Surface|Edge selection, placement-validity hints,
+  lock state (locked label states the tolerance so it can never contradict the
+  rounded readout), and directional hold-to-zero (vector reference, not magnitude
+  subtraction).
 - `feature/ruler`, `feature/tools` — static placeholders.
 
 Not yet implemented (deliberately — see TODO markers):