feat(fft): Blastware-compatible channel FFT (waveform_fft)

channel_spectrum(samples, sps) → the single-sided amplitude spectrum Blastware's
FFT Report draws, and dominant_frequency() picks its peak in the 2–250 Hz band.

Reverse-engineered against 7 BE12844 (MiniMate Plus) events with Blastware FFT
reports as ground truth. Recipe: DC-remove, NO window (a window smears the peak
and worsens the match), zero-pad to 4096 (→ 0.25 Hz bins at 1024 sps — the
resolution every reported dominant frequency lands on), single-sided 2/N
amplitude. Reproduces Blastware's dominant frequency to the exact bin on all
28 channels and the amplitude to report precision.

This is the missing piece for both the USBM RI8507 compliance chart (its scatter
is these (freq, amp) points vs the limit curve) and the FFT view.

Pure numpy, series-agnostic (feed it in/s samples from either decoder). The 7
events land in tests/fixtures as the oracle (force-added past the fixtures
gitignore, matching 5-11-26 / decode-re-5-8-26).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YDXjZCr4RqT2U3QvMDhgzf

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