feat(obs): unified zero_obs control and time-normalised readback

- Replace split zero_force_segment / zero_sensor_segment with unified
  zero_obs_async() — a single memset covers all three obs segments
  (force, torque, sensor), resetting the step accumulator.
- Add _obs_accum_steps counter so read_*(normalize=True) returns the
  physically meaningful per-step average for all telemetry fields.
- Sensor now always applies area-normalisation internally; the normalize
  parameter only controls the additional time-normalisation step.
- run() gains zero_obs=True parameter (default) to control reset-on-step.
- 7 new integration tests covering accumulation, zeroing, and normalise.
- Fix bug in test_sensor_accuracy.py (undefined loop variable i).
- Bump version to 0.4.0 for the API change.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/validation/run_sah04_st_matrix.py

@@ -321,7 +321,7 @@ def run_one_simulation(
             stream.synchronize()
             sim.bodies.download_obs_full_async(stream)
             stream.synchronize()
-            fvec = sim.bodies.read_force(0)
+            fvec = sim.bodies.read_force(0, normalize=False)
             lift = float(fvec[1])
             drag = float(fvec[0])
             if not np.isfinite(lift) or not np.isfinite(drag):