refactor: split lib.rs into params and editor modules

Move parameter structs, defaults, and build_settings into src/params.rs;
move the egui editor into src/editor.rs. lib.rs now holds only the plugin
shell, DSP wiring, and process(). No behavior change (16/16 tests pass).

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

src/dsp/mod.rs

@@ -1,8 +1,8 @@
 //! 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.
+//! The signal chain: a `crossover` filterbank splits into bands, each band (plus the summed
+//! 'All' channel) runs a `compressor`, and a `limiter` (with a true-peak `oversampler` detector)
+//! is the final stage. `biquad` is the shared filter primitive the crossover is built from.
 
 pub mod biquad;
 pub mod compressor;