Stage 3: 3-band LR4 crossover + per-band compressors into the 'All' channel

Splits the input into low/mid/high with a Linkwitz-Riley 24 dB/oct crossover,
compresses each band, sums them, then runs the sum through a fourth 'All'
compressor. Bypassing the three bands collapses the plugin to a simple full-band
comp driven by 'All' (the crossover sums flat in magnitude).

- src/dsp/biquad.rs: generic RBJ biquad (Transposed Direct Form II), LP/HP/AP
- src/dsp/crossover.rs: 3-band LR4 filterbank; lower band all-pass-compensated at
  the higher crossover so the bands sum to flat magnitude (an all-pass, not a
  bit-exact null — that only holds for linear-phase FIR). Mirrors nih-plug's
  crossover plugin design.
- src/lib.rs: 4 Compressor instances (low/mid/high/all) + Crossover; params
  restructured to 4 nested CompressorParams (id_prefix low/mid/high/all) plus
  global crossover_low_hz/crossover_high_hz/look_ahead_ms; 4-column lo|mid|hi|all
  egui UI; latency = two series stages (bands + all), constant, reported once
- 10 unit tests (adds biquad LP/AP magnitude, crossover flat-magnitude
  reconstruction, band-split sanity)
- README: Stage 3 marked done; corrected the 'sum flat' expectation to flat
  magnitude (IIR LR sums to an all-pass, not a time-domain null)

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

README.md

@@ -59,7 +59,7 @@ a first-class mode, not an afterthought.
 ### Crossover Filterbank
 - Linkwitz-Riley 4th-order (LR4) filters at each crossover frequency
 - LR4 = two cascaded biquads (Butterworth LP or HP)
-- Bands sum phase-coherently back to flat
+- Bands sum phase-coherently to flat **magnitude** (the sum is an all-pass; lower bands get an all-pass at each later crossover to match phase — not a bit-exact time-domain null)
 - Crossover frequencies are user-adjustable parameters
 ### Per-Band Compressor
 - Level detection: switchable peak / RMS (RMS window currently hardcoded small; can be exposed later)
@@ -114,9 +114,9 @@ src/
   dsp/
     mod.rs          # ✅ module declarations
     compressor.rs   # ✅ full-band comp: peak/RMS detector, gain computer, ballistics, look-ahead delay
-    crossover.rs    #    (planned) LR4 filterbank (biquad chains)
+    crossover.rs    # ✅ LR4 3-band filterbank with all-pass phase compensation
+    biquad.rs       # ✅ generic biquad (Transposed Direct Form II)
     limiter.rs      #    (planned) output true-peak brickwall limiter
-    biquad.rs       #    (planned) generic biquad (Direct Form II transposed)
     delay.rs        #    (planned) look-ahead delay (currently lives inside compressor.rs)
     oversampler.rs  #    (planned) 4x oversampler for true-peak detection
   editor/
@@ -170,10 +170,12 @@ is essential — without it FL silently skips a plugin it has seen before.)
  
 Work through these stages in order — each stage produces a loadable, audible plugin.
 
-**Status (2026-06-15):** Stages 1–2 are complete. Look-ahead + latency reporting (from Stage 4)
-and a basic slider UI (from Stage 5) were pulled forward and already work. **Next: Stage 3 —
-crossover filterbank.** DSP currently lives in `src/dsp/compressor.rs`; params and the egui
-editor are still inline in `src/lib.rs` (not yet split into `params.rs` / `editor/`).
+**Status (2026-06-17):** Stages 1–3 complete — full-band compressor, peak/RMS detection, and now
+the 3-band LR4 crossover feeding per-band compressors summed into the 'All' channel (4 reusable
+`Compressor` instances). Look-ahead + latency (Stage 4) and a basic 4-column UI (Stage 5) are in.
+**Next: Stage 4 — output brickwall limiter + oversampler.** DSP is in `src/dsp/`
+(`biquad.rs`, `crossover.rs`, `compressor.rs`); params and the egui editor are still inline in
+`src/lib.rs` (not yet split into `params.rs` / `editor/`).
  
 ### Stage 1 — Skeleton plugin ✅
 - [x] NIH-plug "passthrough" compiling and loading in DAW
@@ -186,17 +188,17 @@ editor are still inline in `src/lib.rs` (not yet split into `params.rs` / `edito
 - [x] Implement gain computer (threshold, ratio, soft knee)
 - [x] Implement attack/release envelope (smooth decoupled peak detector)
 - [x] Wire into `process()`; covered by unit tests (static curve, knee continuity, steady state, RMS, constant latency)
-### Stage 3 — Crossover filterbank  ⬅ next
-- [ ] Implement LR4 LP and HP biquad chains in `crossover.rs`
-- [ ] Verify bands sum flat (null test: sum vs dry should be silence)
-- [ ] Add per-band bypass; with all bands bypassed, output must null against dry (proves the "simple comp" mode path)
-- [ ] Apply per-band compressor to each band
-- [ ] Sum bands back together
-- [ ] Run the summed signal through the 'All' channel comp/lim (reuse the per-band compressor) before output
-### Stage 4 — Look-ahead + brickwall limiter  *(look-ahead + latency done early)*
-- [x] Look-ahead delay (circular buffer) — currently inside `compressor.rs`, no separate `delay.rs` yet
+### Stage 3 — Crossover filterbank ✅
+- [x] Implement LR4 LP/HP biquad chains in `crossover.rs` (+ generic `biquad.rs`, Transposed Direct Form II)
+- [x] Verify bands sum flat — for IIR LR4 the sum is an **all-pass** (flat *magnitude*, phase-shifted), not a bit-exact null; lower bands get an all-pass at each later crossover to phase-match. Tested via `bands_sum_to_flat_magnitude`
+- [x] Per-band bypass — a bypassed band passes its delayed dry band; with all three bypassed the 'All' channel sees the flat-magnitude reconstruction = the simple-comp mode
+- [x] Apply per-band compressor to each band
+- [x] Sum bands back together
+- [x] Run the summed signal through the 'All' channel compressor before output
+### Stage 4 — Output brickwall limiter + oversampler  ⬅ next  *(look-ahead + latency already done)*
+- [x] Look-ahead delay (circular buffer) — inside `compressor.rs`, no separate `delay.rs`
 - [x] Wire look-ahead: detector reads N samples ahead of the VCA
-- [x] Report latency — via `context.set_latency_samples()`, reported once as a constant (see Latency note)
+- [x] Report latency — `context.set_latency_samples()` once; now the constant two-stage total (bands + 'All')
 - [ ] Implement `oversampler.rs` (4x, use a polyphase FIR or windowed sinc)
 - [ ] Implement brickwall output limiter with true-peak detection
 ### Stage 5 — Basic egui UI  *(basic version done early)*