Stage 4a: base-rate look-ahead brickwall limiter

Adds the output limiter stage after the 'All' channel. Guarantees the output
never exceeds the ceiling: fixed 1.5 ms look-ahead, stereo-linked sliding-max
peak detection over the look-ahead window -> gain = ceiling/window_max, decoupled
smoothing (fast attack / user release), and a final clamp as the hard guarantee.

- src/dsp/limiter.rs: Limiter (sample-peak; true-peak via oversampling is 4b)
- src/lib.rs: wired as final stage; new globals output_ceiling_db (-24..0) and
  limiter_release_ms; latency now the constant three-stage total (bands+All+limiter);
  two UI sliders added to the global row
- 14 unit tests (4 new: ceiling guarantee on spikes, loud-sine limiting,
  transparency below ceiling, latency)
- README/docs updated (Stage 4 split into 4a done / 4b oversampling)

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

README.md

@@ -73,9 +73,9 @@ a first-class mode, not an afterthought.
 - Bands are individually bypassable; with all three bypassed the (phase-coherent) crossover sum equals the dry input, so the 'All' channel alone acts as a full-band comp/lim
 - Has its own look-ahead; the plugin reports a single **constant** total latency (the fixed band + 'All' look-ahead), set once — see Latency below
 ### Output Limiter
-- True-peak brickwall (ceiling = 0 dBFS or user-defined)
-- 4x oversampling for inter-sample peak detection
-- Short attack (≤ 0.1 ms), auto-release
+- Brickwall, ceiling = 0 dBFS or user-defined (`output_ceiling`). Look-ahead + sliding-max peak detection + a ceiling clamp guarantee the output never exceeds the ceiling
+- Short attack (≤ 0.1 ms), auto-release (release time user-set)
+- **Sample-peak today**; 4x-oversampled true-peak (inter-sample) detection is the remaining Stage-4 work
 ### Latency
 - Reported via `context.set_latency_samples()` in `initialize()` — **never** from `process()`; renegotiating latency mid-stream crashes some hosts (FL included)
 - Reported latency is a **constant** (the max look-ahead); the look-ahead control only moves the detector tap within that fixed delay
@@ -87,6 +87,7 @@ a first-class mode, not an afterthought.
 ### Global
 - `input_gain` — pre-gain before filterbank (dB)
 - `output_ceiling` — brickwall ceiling (dBFS, default 0.0)
+- `limiter_release_ms` — output limiter release time
 - `look_ahead_ms` — look-ahead time (0–5 ms). Reported latency is **constant** (the max look-ahead); the knob only moves the detector tap within that fixed delay, so it is safe to adjust during playback (changing reported latency mid-stream crashes some hosts, FL included)
 - `crossover_low_hz` — low/mid crossover frequency
 - `crossover_high_hz` — mid/high crossover frequency
@@ -116,8 +117,8 @@ src/
     compressor.rs   # ✅ full-band comp: peak/RMS detector, gain computer, ballistics, look-ahead delay
     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
-    delay.rs        #    (planned) look-ahead delay (currently lives inside compressor.rs)
+    limiter.rs      # ✅ look-ahead brickwall limiter (sample-peak; true-peak pending)
+    delay.rs        #    (planned) look-ahead delay (currently inside compressor.rs / limiter.rs)
     oversampler.rs  #    (planned) 4x oversampler for true-peak detection
   editor/
     mod.rs          #    (planned) egui editor split out of lib.rs
@@ -175,12 +176,11 @@ 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 (2026-06-17):** 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/`).
+**Status (2026-06-19):** Stages 1–3 plus the **base-rate brickwall limiter** (Stage 4a) are done:
+3-band LR4 crossover → per-band compressors (peak/RMS) → 'All' channel → look-ahead brickwall
+limiter, with a basic 4-column UI. **Next: Stage 4b — 4× oversampling for true-peak (inter-sample)
+limiting** (deferred as the CPU-heavy part). DSP is in `src/dsp/` (`biquad.rs`, `crossover.rs`,
+`compressor.rs`, `limiter.rs`); params and the egui editor are still inline in `src/lib.rs`.
  
 ### Stage 1 — Skeleton plugin ✅
 - [x] NIH-plug "passthrough" compiling and loading in DAW
@@ -200,12 +200,13 @@ the 3-band LR4 crossover feeding per-band compressors summed into the 'All' chan
 - [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`
+### Stage 4 — Output brickwall limiter + oversampler  *(4a done; 4b = oversampling)*
+- [x] Look-ahead delay (circular buffer) — inside `compressor.rs` and `limiter.rs`, no separate `delay.rs`
 - [x] Wire look-ahead: detector reads N samples ahead of the VCA
-- [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
+- [x] Report latency — `context.set_latency_samples()` once; now the constant three-stage total (bands + 'All' + limiter)
+- [x] Brickwall output limiter (`limiter.rs`): look-ahead + sliding-max peak detect + ceiling clamp guarantee; limits **sample** peaks
+- [ ] `oversampler.rs` (4x, polyphase FIR / windowed sinc)  ⬅ next
+- [ ] True-peak (inter-sample) limiting on top of the brickwall, via the oversampler
 ### Stage 5 — Basic egui UI  *(basic version done early)*
 - [x] Add `nih_plug_egui` editor
 - [x] Sliders for all current parameters (`ParamSlider` grid)