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>

setup.py

@@ -5,7 +5,7 @@ with open("README.md", "r", encoding="utf-8") as fh:
 
 setup(
     name='CelerisLab',
-    version='0.3.0',
+    version='0.4.0',
     author='Frank14f',
     description='GPU-accelerated Lattice Boltzmann Method (LBM) CFD solver using CUDA',
     long_description=long_description,