Add ARM64/Apple Silicon support

[saidatom]

Problem

The library only works on Linux x86-64 and fails on ARM64 systems including:

• Apple Silicon Macs (M1/M2/M3/M4)
• ARM-based Docker/DDEV containers
• ARM64 Linux servers

Error

Failed to load publiccode-parser library

Root Cause

Only one binary is shipped: lib/libpubliccode-parser.so (x86-64 Linux)

The library needs binaries for:

• Linux ARM64
• macOS Intel (x86-64)
• macOS Apple Silicon (ARM64)

Solution Needed

1. Build binaries for all architectures in CI
2. Update src/Parser.php to detect platform and load correct binary
3. Ship all binaries with the package
4. Update README to reflect multi-platform support

Files to Modify

• Makefile - add cross-compilation targets
• src/Parser.php:150-169 - add platform detection
• .github/workflows/packagist.yml - build all architectures
• README.md:13 - update supported platforms

Related

The Go library already supports cross-compilation, so this is mainly a build/distribution issue.