Scaffold NIH-plug project: full-band gain VST3/CLAP with one-slider egui GUI

Sets up the Rust/NIH-plug build (pinned to nih-plug rev f36931f) producing
VST3 and CLAP bundles via 'cargo xtask bundle'. Implements the first landmark:
a full-band gain plugin with a single egui ParamSlider.

- Cargo workspace + xtask bundler, .cargo alias, bundler.toml
- src/lib.rs: Codename206 plugin (gain param, egui editor)
- deploy.ps1/.bat: build + install to system VST3/CLAP folders (real copy,
  not a junction, for FL Studio); -User flag for a no-admin dev loop
- LICENSE: GPL-3.0 (required by NIH-plug's VST3 bindings)
- README: corrected architecture for the 'All' aggregate channel (4th comp/lim
  stack on the summed bands; all-bands-bypassed = simple full-band comp)
- .gitignore excludes /target and the local nih-plug + _template reference clones

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

README.md

@@ -10,6 +10,7 @@ Built with **Rust** + **NIH-plug** (VST3 + CLAP output) + **egui** for the UI.
  
 **Goals:**  
 - 3-band (configurable crossover points) compressor/limiter  
+- An 'All' aggregate channel: a 4th comp/lim stack on the summed bands, so bypassing all bands turns the plugin into a simple full-band compressor (mirrors FL's Maximizer)  
 - Look-ahead brickwall output limiter with true-peak detection  
 - Real-time gain reduction metering per band  
 - Custom gain curve visualiser  
@@ -33,13 +34,23 @@ Built with **Rust** + **NIH-plug** (VST3 + CLAP output) + **egui** for the UI.
 ```
 Input
   └─ Crossover filterbank (Linkwitz-Riley LR4 @ each crossover freq)
-       ├─ Band 1 (low)   → look-ahead delay → compressor VCA → gain stage
-       ├─ Band 2 (mid)   → look-ahead delay → compressor VCA → gain stage
-       └─ Band 3 (high)  → look-ahead delay → compressor VCA → gain stage
-            └─ Sum → output brickwall limiter (true-peak, 4x oversampled) → output
+       ├─ Band 1 (low)   → look-ahead delay → compressor VCA → gain stage ─┐ (bypassable)
+       ├─ Band 2 (mid)   → look-ahead delay → compressor VCA → gain stage ─┤ (bypassable)
+       └─ Band 3 (high)  → look-ahead delay → compressor VCA → gain stage ─┤ (bypassable)
+                                                                           │
+       Sum of bands ◄──────────────────────────────────────────────------┘
+         └─ 'All' channel → look-ahead delay → compressor VCA → gain stage
+              └─ output brickwall limiter (true-peak, 4x oversampled) → output
 ```
  
 The detector for each band reads `look_ahead_ms` ahead of the VCA, so gain reduction is already ramping when the transient arrives.
+
+**The 'All' aggregate channel** (mirrors FL's Maximizer): the three bands are summed and the
+result passes through a *fourth*, full-band compressor/limiter stack before the output limiter.
+Because the LR4 filterbank sums phase-coherently flat, **bypassing all three bands leaves the
+summed signal identical to the input** — so the plugin collapses into a plain single-band
+compressor/limiter driven entirely by the 'All' channel. That makes "multiband off = simple comp"
+a first-class mode, not an afterthought.
  
 ---
  
@@ -56,6 +67,11 @@ The detector for each band reads `look_ahead_ms` ahead of the VCA, so gain reduc
 - Attack / release envelopes (logarithmic ballistics)
 - Makeup gain per band
 - Look-ahead: circular delay buffer on the audio path; detector reads ahead
+### 'All' Aggregate Channel
+- Structurally **identical to a per-band compressor** — reuse the same comp/lim code/params, just fed the summed signal instead of a filtered band
+- Runs after the three bands are summed, before the output brickwall limiter
+- 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 delay; total reported latency = max(band look-ahead) + 'All' look-ahead
 ### Output Limiter
 - True-peak brickwall (ceiling = 0 dBFS or user-defined)
 - 4x oversampling for inter-sample peak detection
@@ -73,14 +89,16 @@ The detector for each band reads `look_ahead_ms` ahead of the VCA, so gain reduc
 - `look_ahead_ms` — look-ahead time (0–10 ms)
 - `crossover_low_hz` — low/mid crossover frequency
 - `crossover_high_hz` — mid/high crossover frequency
-### Per Band (× 3, use a `#[nested]` params struct)
+### Per-Channel Compressor (× 4: low, mid, high, **all** — one `#[nested]` params struct reused)
 - `threshold_db`
 - `ratio` — 1.0 (off) to ∞ (limiting)
 - `attack_ms`
 - `release_ms`
 - `knee_db` — soft knee width
 - `makeup_gain_db`
-- `bypass` — per-band bypass
+- `bypass` — per-channel bypass (bypassing low+mid+high = simple full-band comp via the 'all' channel)
+
+The 'all' channel uses the same struct so its UI and DSP are identical to a band; it just sits after the band sum.
 ---
  
 ## Project Structure
@@ -109,22 +127,38 @@ src/
  
 ## Build Steps
  
-```bash
-# Install Rust (if not already)
-curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh
- 
-# Clone NIH-plug cookiecutter or start fresh
-cargo new --lib my_maximizer
-cd my_maximizer
- 
-# Add dependencies to Cargo.toml:
-# nih-plug = { git = "https://github.com/robbert-vdh/nih-plug", features = ["assert_process_allocs"] }
-# nih-plug-egui = { git = "https://github.com/robbert-vdh/nih-plug" }
- 
-# Build and bundle
-cargo xtask bundle my_maximizer --release
-# Output: target/bundled/my_maximizer.vst3
+The project is already scaffolded (NIH-plug + nih_plug_egui, pinned to a fixed git rev in
+`Cargo.toml`). You do **not** need the Steinberg VST3 SDK — NIH-plug bundles its own bindings.
+
+**Prerequisites (Windows):**
+- Rust stable (`rustup` — `winget install Rustlang.Rustup`)
+- Visual Studio 2022 with the "Desktop development with C++" workload (provides the MSVC linker)
+
+```powershell
+# Build + bundle the VST3 and CLAP
+cargo xtask bundle codename_206 --release
+# Output: target\bundled\Codename 206.vst3  and  Codename 206.clap
 ```
+
+### Deployment
+
+FL Studio scans `C:\Program Files\Common Files\VST3` by default, **ignores directory junctions**
+(so a symlinked bundle is invisible to its scanner), and caches failed scans. So deployment must
+copy a *real* bundle into a folder FL scans, then FL must be told to rescan failed plugins.
+
+Use the provided script (no need to remember the details):
+
+```powershell
+.\deploy.ps1            # build, then copy to the global VST3/CLAP folders (one UAC prompt)
+.\deploy.ps1 -SkipBuild # reinstall the last build without rebuilding
+.\deploy.ps1 -User      # copy to %LOCALAPPDATA%\Programs\Common\VST3 instead (no admin) — best for a dev loop
+```
+
+`deploy.bat` is a double-click wrapper around the same script.
+
+After deploying, in FL Studio: **Options → Manage plugins → tick "Rescan previously failed
+plugins" → Find installed plugins**, then search for **Codename 206**. (The rescan-failed step
+is essential — without it FL silently skips a plugin it has seen before.)
  
 ---
  
@@ -146,8 +180,10 @@ Work through these stages in order — each stage produces a loadable, audible p
 ### Stage 3 — Crossover filterbank
 - [ ] 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 to output
+- [ ] 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
 - [ ] Implement `delay.rs` circular buffer
 - [ ] Wire look-ahead: detector reads N samples ahead of VCA