Stage 2: full-band compressor with constant-latency look-ahead

Implements the single full-band feed-forward compressor (the engine that will be
reused per band + for the 'All' channel). Design follows Giannoulis et al. 2012:
log-domain gain computer with a quadratic soft knee feeding a smooth decoupled
peak detector for attack/release ballistics. Stereo-linked peak detection.

Look-ahead uses a fixed audio delay with a constant reported latency (set once in
initialize); the knob only moves the detector tap within that delay. This avoids
renegotiating latency from process(), which crashed FL Studio when the look-ahead
was adjusted during playback.

- src/dsp/compressor.rs: Compressor + CompressorSettings, RT-safe (no alloc in
  process; buffers sized in prepare; envelope denormals flushed in-code)
- src/lib.rs: nested CompressorParams (threshold/ratio/knee/attack/release/makeup/
  bypass) + global look-ahead; egui ParamSlider grid; latency reported once
- 5 unit tests (static curve, knee continuity, steady-state convergence, constant
  latency); Cargo.toml lib crate-type added so tests link
- README: 'All' channel architecture already documented; look-ahead spec updated

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/dsp/mod.rs

@@ -0,0 +1,7 @@
+//! DSP building blocks for Codename 206.
+//!
+//! Stage 2 introduces the full-band compressor (also the engine that will be reused
+//! per band and for the 'All' aggregate channel — see README.md). Later stages add the
+//! crossover filterbank, output limiter, and oversampler alongside it.
+
+pub mod compressor;