Update butter zero phase with composition of standard forward and a new backward filter

[cboulay]

• Replaces the non-streaming filtfilt/sosfiltfilt implementation with a two-stage CompositeProcessor: a forward causal ButterworthFilterTransformer followed by a new ButterworthBackwardFilterTransformer that buffers and applies the filter in reverse, making the zero-phase filter fully streaming-compatible
• Adds settle_cutoff and max_pad_duration settings that compute pad length from the filter's actual impulse response settling time, replacing the old padtype/padlen parameters which relied on scipy's order-based heuristic (inadequate for filters with very low normalized frequencies, e.g. a 30 Hz lowpass at 30 kHz needs 2,292 samples to settle but scipy's heuristic gives only 15)
• Significantly expands test coverage with 65 unit tests and 1 integration test covering pad length computation, warmup behavior, streaming chunked processing, axis position handling, zero-phase property verification, and the new settings parameters