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>

Cargo.toml

@@ -14,7 +14,8 @@ license = "GPL-3.0-or-later"
 members = ["xtask"]
 
 [lib]
-crate-type = ["cdylib"]
+# `cdylib` is the plugin binary; `lib` (rlib) lets `cargo test` link the unit tests.
+crate-type = ["cdylib", "lib"]
 
 [dependencies]
 # Pinned to the exact commit of the local ./nih-plug reference clone so the build