Extend the RDS stack with TP/TA live updates, CT, RT+, PTYN, AF and EON support, and add UI/config controls for the new RDS feature set.

Add experimental single-sideband and vestigial-sideband stereo encoding
modes alongside the standard DSB-SC stereo subcarrier path.

- add encoder mode selection (DSB, SSB, VSB)
- add Hilbert-transform based quadrature path for SSB generation
- add vestigial split with low-band DSB and high-band SSB handling
- wire the mode through config, control, engine, generator, and fmrtx
- add test coverage for the SSB path

The default mode remains the standard DSB stereo encoder.

Complete redesign of the audio watermark system. The previous time-domain
PN chip correlator failed over the FM air channel due to non-linear clock
drift between TX and RX sample clocks, destroying bit-boundary alignment
and producing ~50% BER (random).

The new system embeds the watermark in STFT magnitudes (frequency domain),
eliminating all sample-clock sensitivity. Based on:

 D. Kirovski, H.S. Malvar, "Spread-Spectrum Watermarking of Audio Signals"
 IEEE Trans. Signal Processing, Vol. 51, No. 4, April 2003

Architecture:

 Encoder (generator.go, two-pass):
 Audio → Stages 1-3 (LPF, drive, clip, cleanup)
 → Decimate ÷19 → 12 kHz mono
 → STFT (512-point Hann, hop=256, 21ms frames)
 → Magnitude × (1 ± 0.059) per PN chip (0.5 dB, multiplicative)
 → ISTFT → difference → ZOH ×19 → interpolation LPF@5.5kHz
 → add to L/R → Stages 4-6 (stereo encode, composite clip, pilot, RDS)

 Decoder (wmdecode, key-free):
 Recording → LPF@5.5kHz → decimate to 12 kHz
 → STFT (same parameters)
 → Cepstrum filter (DCT, zero first 8 coefficients → -6 dB carrier noise)
 → Cycle-offset search (6400 candidates, ~70s for 20-min recording)
 → PN correlation → 128 soft bit decisions
 → RS(16,8) Vandermonde decode → 64-bit fingerprint

Key properties:

 - Multiplicative embedding: watermark scales with audio content.
 Loud audio masks stronger watermark, silence gets zero watermark.
 No gate needed (old system required silence gate to prevent audibility).
 - Block repetition R=5: each PN chip repeated across 5 consecutive
 STFT frames. Detection uses center frame only. Tolerates ±2 frames
 (±43ms) of timing drift without any clock recovery.
 - Fixed PN sequence: spreading code is public (seed "fmrtx-stft-pn-v1"),
 enabling blind fingerprint extraction without knowing the license key.
 Key identity is carried solely in the RS-encoded payload.
 - PCC covert channel: PN partitioned into 128 subsets (one per data bit),
 permuted across the watermark cycle for localized-damage resilience.

Parameters:

 WMRate: 12000 Hz (228000/19, 192000/16 — exact both sides)
 FFT: 512-point, Hann window, hop=256 (50% overlap)
 Sub-band: bins 9-213 (200-5000 Hz), 204 frequency chips/frame
 Embedding: 0.5 dB (multiplicative, 0.00 dB RMS change on audio)
 Spreading: 204 bins × 10 groups/bit = 2040 chips/bit (33 dB gain)
 Block rep: R=5 (±2 frame drift tolerance)
 Payload: RS(16,8) → 64 bit fingerprint (SHA-256 truncated)
 WM cycle: 136.5 seconds
 Decode margin: ~19 dB over noise floor at 0.5 dB embedding

Over-the-air results (PlutoSDR TX → SDRplay RX, 102.8 MHz):

 BER: 0/128
 Erasures: 0
 avg|c|: 2260
 Spectrum: clean, no artifacts above 6 kHz

Bug fixes included:

 - RS decoder: replaced Forney formula (used α^pos instead of α^(15-pos)
 due to polynomial convention mismatch) with Vandermonde Gaussian
 elimination solver. The Horner-method syndrome computation uses
 C(x) = c[0]x^15 + ... + c[15], mapping byte position j to polynomial
 power (15-j). Forney was using α^j as the error locator instead of
 α^(15-j), producing wrong correction magnitudes for all erasure
 configurations.

 - Generator decimation: anti-alias LPF was applied only to every 19th
 sample instead of all composite-rate samples. IIR filter state was
 updated only at 12 kHz effective rate, producing incorrect filtering.
 Fixed: LPF processes all 228 kHz samples, then decimate.

 - ZOH spectral images: zero-order hold upsample (12k→228k) created
 images at 12k, 24k, 36k Hz, leaking into pilot/stereo-sub/RDS bands.
 Fixed: separate interpolation LPF@5.5kHz on the upsample path.

 - Detect() single-cycle bug: iterated over groups (one frame per group)
 instead of all recording frames. Longer recordings did not improve
 SNR. Fixed: iterate all frames with modular wrapping for automatic
 multi-cycle averaging.

New files:
 internal/dsp/fft.go Radix-2 Cooley-Tukey FFT/IFFT
 internal/watermark/stft_watermark.go STFT embedder + detector
 internal/watermark/stft_roundtrip_test.go

Changed files:
 internal/watermark/watermark.go RS Vandermonde fix, accessor methods
 internal/offline/generator.go Two-pass architecture, STFT overlay
 cmd/wmdecode/main.go Complete rewrite, key-free extraction
 cmd/fmrtx/main.go Remove old watermark log + import

Removed:
 Old time-domain Embedder, gate, pulse-shaping LPF, PN sequence,
 ChipRate/Level/RecordingRate constants, DiagnosticState

Add an STFT watermark path inspired by Kirovski & Malvar, including the frequency-domain embedder/decoder, FFT support, and round-trip coverage. Wire the generator and CLI tools to use the new analysis/synthesis flow for watermark experiments on the watermark-rework branch.

Wire tone frequency, tone amplitude, and audio gain through the live control path so the UI's live-update behavior matches the engine.

This changes the generator live params to carry tone and gain values, propagates them through Engine.UpdateConfig and txBridge.UpdateConfig, and extends the control-plane patch types accordingly.

It also refines the control API behavior:
- avoid holding the server config mutex across tx.UpdateConfig()
- report live=true only when a request contains at least one genuinely live-applicable field

Together these fixes align the UI semantics with the actual runtime behavior and remove a lock hazard in the config update path.

Add a lock-free stdin PCM ingest path, streaming resampler, stereo-linked limiting and pre-MPX audio filtering, plus the engine/control wiring needed to drive live audio into TX mode. Also document the ingest API and include a helper batch script for piping ffmpeg audio into fmrtx.

All major TX parameters are now hot-swappable during transmission:
- DSP params (drive, stereo, pilot, RDS levels, limiter) via
 atomic.Pointer[LiveParams], loaded once per chunk (~50ms)
- RDS text (PS, RT) via atomic.Value in encoder, applied at
 RDS group boundaries (~88ms)
- TX frequency via driver.Tune(), applied between chunks

Zero locks in DSP path. HTTP handler writes atomics, run loop
reads them. Validation happens before store.

New: SoapyDriver.Tune() for live frequency changes.
New: LiveConfigUpdate/LivePatch types for type-safe patching.
New: 4 engine tests for live DSP/freq/RDS updates + validation.

Clarify that outputDrive controls only the composite signal path while PlutoSDR hardware gain stays fixed at 0 dB. Relax outputDrive validation to allow stronger composite drive during hardware tuning and update the Pluto example config accordingly.

Introduce CLI TX mode, hardware driver selection, IQ resampling to device rate, and platform-specific PlutoSDR/SoapySDR integrations. Update engine pacing for blocking hardware writes and refresh docs/release notes for the hardware-ready v0.6.0-pre milestone.