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_kan99b_rotating_cylinder.py

@@ -314,7 +314,7 @@ def _run_one(
             stream.synchronize()
             sim.bodies.download_obs_full_async(stream)
             stream.synchronize()
-            force = sim.bodies.read_force(0)
+            force = sim.bodies.read_force(0, normalize=False)
             fx = float(force[0])
             fy = float(force[1])
             if not np.isfinite(fx) or not np.isfinite(fy):