Add indications for cross building

BUILDING.md

@@ -0,0 +1,91 @@
+# Building Cagire
+
+## Prerequisites
+
+**Rust** (stable toolchain): https://rustup.rs
+
+## System Dependencies
+
+### macOS
+
+```bash
+brew install cmake
+```
+
+cmake is required by `rusty_link` (Ableton Link C++ bindings). Xcode Command Line Tools provide the C++ compiler. CoreAudio and CoreMIDI are built-in. The desktop build needs no additional dependencies on macOS (Cocoa/Metal are provided by the system).
+
+### Linux (Debian/Ubuntu)
+
+```bash
+sudo apt install cmake g++ pkg-config libasound2-dev libjack-jackd2-dev
+```
+
+For the desktop build (egui/eframe), also install:
+
+```bash
+sudo apt install libgl-dev libxkbcommon-dev libx11-dev libxcursor-dev libxrandr-dev libxi-dev libwayland-dev
+```
+
+### Linux (Arch)
+
+```bash
+sudo pacman -S cmake gcc pkgconf alsa-lib jack2
+```
+
+For the desktop build:
+
+```bash
+sudo pacman -S libxkbcommon libx11 libxcursor libxrandr libxi wayland mesa
+```
+
+### Linux (Fedora)
+
+```bash
+sudo dnf install cmake gcc-c++ pkgconf-pkg-config alsa-lib-devel jack-audio-connection-kit-devel
+```
+
+For the desktop build:
+
+```bash
+sudo dnf install libxkbcommon-devel libX11-devel libXcursor-devel libXrandr-devel libXi-devel wayland-devel mesa-libGL-devel
+```
+
+### Windows
+
+Install Visual Studio Build Tools (MSVC) and CMake. Everything else is provided by the Windows SDK.
+
+## Build
+
+Terminal (default):
+
+```bash
+cargo build --release
+```
+
+Desktop (egui window):
+
+```bash
+cargo build --release --features desktop --bin cagire-desktop
+```
+
+## Run
+
+Terminal (default):
+
+```bash
+cargo run --release -- [OPTIONS]
+```
+
+Desktop (egui window):
+
+```bash
+cargo run --release --features desktop --bin cagire-desktop
+```
+
+| Flag | Description |
+|------|-------------|
+| `-s, --samples <path>` | Sample directory (repeatable) |
+| `-o, --output <device>` | Output audio device |
+| `-i, --input <device>` | Input audio device |
+| `-c, --channels <n>` | Output channel count |
+| `-b, --buffer <size>` | Audio buffer size |