Implement precision timing recovery functions:
- _refine_symbol_boundary(): Scans at 1/32-symbol resolution to find
exact chirp boundary by maximizing dechirped SNR
- _find_sfd_boundary(): FFT-based correlation with downchirp template
to find exact data start position
Bug fixes:
- Fix _is_downchirp() false positives by comparing both correlations
- Fix _estimate_cfo() to return values in [0, N) range
The improved sync_from_samples() now produces bins identical to the
reference lora_decode_gpu decoder.
- Add Channelizer class for wideband capture processing (2 MHz → 125 kHz)
- FIR low-pass filter with scipy.firwin (or fallback windowed-sinc)
- Proper decimation for anti-aliasing
- Fix FrameSync preamble detection to accept any CFO
- Real captures have significant carrier frequency offset
- Preamble bins appear at arbitrary values, not just near 0
- Now accepts any strong signal as first preamble, validates consistency
- Add decode_capture.py example script for processing raw BladeRF captures
- PHYDecode verified to match existing lora_phy decoder output
