chore: set up GitHub Actions CI matrix for all platforms

[digital-michael]

Goal

Establish CI that builds and tests on all supported platforms before any code changes land in dev.

Platforms required

• Linux x86-64 (Fedora / Ubuntu) — X11 and Wayland targets
• Windows (MSVC or MinGW)
• macOS (clang/AppleClang)

Acceptance criteria

• .github/workflows/ci.yml created on a feature/ci-matrix branch
• Build matrix covers all 3 platforms
• CI runs on every PR targeting dev or phase/*
• Zero failing jobs on unmodified fork (green baseline)
• Self-hosted runners registered for macOS and Linux/Wayland (see chore: verify local build on Fedora 43 / Wayland (unmodified fork) #5)

Notes

CI must be green before Phase 0 is closed. Performance baseline (see #7) also runs in CI.