Add cross-compilation support for Linux and Windows

- Add Docker-based build system for cross-compiling on Apple Silicon
- Create Dockerfiles for Linux x86_64 and ARM64, Windows x86_64
- Add build-all.sh script for automated multi-platform builds
- Configure cargo-deb and cargo-generate-rpm for Linux packaging
- Update README with cross-compilation instructions

Supported outputs:
- Linux x86_64: .deb, .rpm, .tar.gz
- Windows x86_64: .zip

Note: Linux ARM64 disabled due to upstream zune-jpeg NEON bug

Cross.toml

@@ -0,0 +1,19 @@
+[build]
+# Don't mount host's cargo/rustup - use container's toolchain
+pre-build = []
+
+[build.env]
+passthrough = ["RUST_BACKTRACE"]
+
+[target.x86_64-unknown-linux-gnu]
+image = "avif-maker-linux-x86_64"
+# Use container's Rust instead of host's
+env.volumes = []
+
+[target.aarch64-unknown-linux-gnu]
+image = "avif-maker-linux-arm64"
+env.volumes = []
+
+[target.x86_64-pc-windows-gnu]
+image = "avif-maker-windows-x86_64"
+env.volumes = []