Add macOS arm64 build

[rakleed]

Closes #257

I checked the compiled build for arm64, and it works correctly.

Here's how Claude Code explained why these changes will be enough:

Details

Dependency analysis and fixes

Dependencies without ARM64 wheels on macOS:

Package	Status	Resolution
imgui==2.0.0	❌ x86_64 only	pip automatically builds from source (sdist contains pre-generated C files, Cython not required)
zipfile-deflate64-macos==0.1.0	⚠️ universal2 for cp39 only	pip builds from source on Python 3.12 (same as on Intel)

All other dependencies (zstd, bencode2, pillow-avif-plugin, lxml, cx-Freeze, etc.) already have native ARM64 or universal2 wheels.

Changes made to .github/workflows/build.yml:

1. Added new runner macos-15 (Apple Silicon / ARM64) to the build matrix
2. Added artifact field to each matrix entry for unique artifact names: Windows, Linux, macOS, macOS-ARM64 — replacing the previous runner.os which produced the same name macOS for both macOS builds

Why this works:

On the ARM64 runner (macos-15), pip won't find compatible wheels for imgui==2.0.0 (x86_64 only) and will automatically fall back to building from sdist. The sdist already contains pre-generated C files, and Xcode Command Line Tools (pre-installed on GitHub runners) will compile them into a native ARM64 binary.

[cantunborn]

This is working. Tested on my fork. Also improves launch time significantly.