feat: /api/iq_capture + /api/record_analyze signal diagnostics

[hunterhubble]

Summary

Adds two RX diagnostic endpoints so a customer can capture and characterise a device's signal against protocol expectations, without needing the live GUI.

1. GET /api/iq_capture?seconds=N — records N s (1–60) of raw IQ forward from now and returns a .npy (complex64) download with X-Sample-Rate-Hz / X-Center-Freq-Hz / X-N-Samples headers.

2. GET /api/record_analyze?seconds=N — records N s (1–30) and returns a plain-text diagnostic report file the customer sends back. For the representative decoded packet it prints, per symbol:

• Timing — duration, inter-symbol gap, and clock drift vs the ideal 8 ms/0.8 ms grid.
• Frequency / channel hopping — dominant tone per symbol vs the expected (calibrated) channel window, with a PASS/FAIL count.
• Amplitude — per-symbol RMS dBFS and SNR above the inter-symbol noise floor.

Plus decoder-reported chipset / synth-res, frequency offset, and Reed-Solomon correction counts, and a one-line summary per packet.

Scope / design

• Decoded packets only for now — the report characterises devices that decode locally (the common bench case). Diagnosing packets that don't decode is deliberately left for a later iteration.
• Values are printed as-measured — no timing tolerances or margin verdicts (the only pass/fail is the channel-window check, whose tolerances are defined by the protocol).
• Reuses existing DSP — correct_symbol_edges for symbol edges; shared helpers were factored into timing.py (dominant_symbol_freq, symbol_amplitudes_dbfs) and config.py (packet_symbol_grid, rotated_hop_sequence) and are now also called by spectrogram.py / processor.py, removing pre-existing duplication.
• Analysis runs in the Flask handler on a copied buffer (like iq_capture), so the live spectrogram isn't stalled; capture is serialised by a shared lock.

Files

• src/stream_web/analysis.py (new) — the diagnostic pipeline (detection → per-symbol analyses → report).
• src/stream_web/app.py — iq_capture + record_analyze routes, shared _capture_iq helper.
• src/stream_web/{timing,config,spectrogram,processor}.py — shared symbol-DSP / geometry helpers.
• CLAUDE.md — endpoint docs.

Testing

• ruff check src/ clean; mock test suite 12/12 pass.
• Verified end-to-end on a recorded capture: per-symbol tables render, channel validation 60/60 in-window.

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