Address a set of production-facing edge cases discovered during bug hunting.

Included fixes:
- make FrameQueue close handling race-safe by replacing the TOCTOU close check with a dedicated close signal channel
- relax tone frequency validation when tone amplitude is zero, and default tone amplitude to 0 to avoid unintended test-tone output
- validate PI codes consistently whenever provided, and require a PI when RDS is enabled
- harden Icecast reconnect backoff against duration overflow
- prevent duplicate hard-reload goroutines from rapid repeated ingest-save requests
- clamp BS.412 power accumulation against negative float drift before sqrt to avoid NaN gain propagation

These changes focus on shutdown safety, config correctness, reconnect robustness, and long-running DSP stability.

Introduce an optional ITU-R BS.412 MPX power limiter, tighten the low-pass/notch filter design around the protected composite path, and document the full DSP architecture and recommended Pluto configuration in detail.