Add comprehensive test suite (36 tests) for the ADR-013 commodity sensing
module covering all components: RingBuffer, SimulatedCollector determinism,
feature extraction (time-domain stats, FFT spectral analysis, band power
isolation), CUSUM change-point detection, presence/motion classification,
and end-to-end CommodityBackend pipeline.
Fix feature_extractor.py: add missing _trim_to_window method that caused
AttributeError on the WifiSample extraction path, add post-trim sample
count guard, and handle constant-signal edge case in skewness/kurtosis
computation to prevent scipy RuntimeWarning.
https://claude.ai/code/session_01Ki7pvEZtJDvqJkmyn6B714
