Phase 1-4: Audio stack, mixer engine, MIDI, and network API

P2-R1: ALSA + JACK2 low-latency config (scripts, quirks, tuning)
P2-R2: Carla integration (build scripts, 8ch rack config, NAM LV2 support)
P2-R3: Plugin manager, categories, blacklist, NAM model support
P3-R1: Mixer DSP engine (channel strip, routing matrix, bus mgr, automation)
P4-R1: MIDI engine (learn mode, clock sync, HID discovery, mapping store)
P4-R2: Network API (OSC server, FastAPI REST, WebSocket, auth, rate limiter)
P5-R1: Touchscreen UI evaluation + main entry point
docs: Audio stack, Carla integration, MIDI support, UI evaluation
tests: Full test suite (292 passing)

docs/touchscreen-ui-evaluation.md

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+# Touchscreen UI Toolkit Evaluation
+
+Raspberry Pi 4B, official 7" (800×480) or Waveshare 5" (720×720) DSI/HDMI
+touch displays. Requirements: faders, meters, mute/solo, routing matrix,
+plugin chain, touch gestures, DPI-aware layout.
+
+## Candidates
+
+### LVGL (Light and Versatile Graphics Library)
+- **Type**: C library, embedded-first, Python bindings via `lvgl` package
+- **RPi support**: Native framebuffer + SDL2/KMS backends, no X11 required
+- **Touch**: Native input device support (libinput/evdev), multi-touch, gestures
+- **DPI**: Pixel-based (px), no automatic scaling — manual math required
+- **Widgets**: Sliders, buttons, labels, charts, tables — good but basic
+- **Performance**: ⭐⭐⭐⭐⭐ — C rendering, 30-60 FPS on RPi 4
+- **Python ergonomics**: ⭐⭐ — Python bindings are auto-generated, not idiomatic;
+  callbacks are C-style, error messages opaque; limited docs for Python usage
+- **Verdict**: Best performance, but Python API is friction-heavy. Good for C
+  projects. Too much pain for a Python codebase.
+
+### GTK 3/4
+- **Type**: C library, GObject introspection for Python (`PyGObject`)
+- **RPi support**: Requires X11 or Wayland compositor — adds RAM overhead
+- **Touch**: Mouse-emulated touch; not natively touch-optimized
+- **DPI**: CSS-based theming, but DPI scaling is complex on small screens
+- **Widgets**: Very rich (sliders, buttons, tree views, notebooks)
+- **Performance**: ⭐⭐ — X11/Wayland overhead, not optimized for embedded
+- **Python ergonomics**: ⭐⭐⭐ — GObject patterns leak through; verbose
+- **Verdict**: Desktop toolkit forced onto embedded. Heavy, poor touch UX.
+  Best for desktop companion apps, not the primary mixer UI.
+
+### SDL2
+- **Type**: C library, Python via `PySDL2`
+- **RPi support**: Framebuffer/KMSDRM backends, no X11 required
+- **Touch**: Raw input events only — no gesture recognition built in
+- **DPI**: None — raw pixels, manual scaling required
+- **Widgets**: None — you build every widget from primitives (rects, lines, text)
+- **Performance**: ⭐⭐⭐⭐ — low-level, fast rendering
+- **Python ergonomics**: ⭐ — no widgets, no layout engine, no theme system.
+  Building a mixer UI in raw SDL2 means writing a UI toolkit from scratch.
+- **Verdict**: Great for games, terrible for complex UIs. Too much boilerplate.
+
+### Kivy (selected)
+- **Type**: Python-native UI framework, OpenGL ES 2.0 backend
+- **RPi support**: Native RPi support via `kivy-rpi`; framebuffer or X11 backends;
+  works with official 7" and Waveshare displays out of the box
+- **Touch**: First-class multi-touch, gesture recognition (swipe, pinch, long-press),
+  mouse fallback for development
+- **DPI**: `dp()` and `sp()` (density-independent pixels/scaled pixels) —
+  automatic scaling across 5" / 7" / desktop displays
+- **Widgets**: Sliders, buttons, toggles, labels, layouts, screens, tabbed panels
+- **Performance**: ⭐⭐⭐ — OpenGL-accelerated; 30-60 FPS on RPi 4 for
+  typical mixer UI complexity; GPU handles rendering, CPU free for DSP
+- **Python ergonomics**: ⭐⭐⭐⭐⭐ — Pure Python, declarative KV language or
+  programmatic widget construction; natural fit for the existing Python codebase
+- **Verdict**: Best balance of performance, touch UX, and Python ergonomics.
+  Purpose-built for multi-touch interfaces. DPI-aware by default. Rich widget
+  set plus easy custom widget creation for faders/meters.
+
+## Decision: Kivy
+
+| Criterion | LVGL | GTK | SDL2 | Kivy |
+|-----------|------|-----|------|------|
+| Touch UX | ★★★★ | ★★ | ★ | ★★★★★ |
+| Performance | ★★★★★ | ★★ | ★★★★ | ★★★ |
+| Python API | ★★ | ★★★ | ★ | ★★★★★ |
+| DPI scaling | ★ | ★★★ | ★ | ★★★★★ |
+| Widget richness | ★★★ | ★★★★★ | ★ | ★★★★ |
+| RPi setup | ★★★★ | ★★ | ★★★ | ★★★★ |
+
+Kivy is the clear winner for a Python-project building a touch-optimized mixer
+UI on Raspberry Pi. The DPI-aware layout system means the same code works on
+5" and 7" displays without manual scaling math. Multi-touch and gestures
+come free. Custom widgets (faders, meters, knobs) are straightforward to
+build on Kivy's Canvas primitives.
+
+### Installation (RPi)
+```bash
+sudo apt install python3-kivy  # system package, includes RPi deps
+# or
+pip3 install kivy  # + manual SDL2/OpenGL deps
+```
+
+### Development (desktop)
+```bash
+pip3 install kivy
+# Run with: python3 main_touch.py
+# Mouse works as touch fallback
+```