Add GitHub Actions CI workflow for fuzz testing

Runs on push to main, nightly at 2am UTC, and manual dispatch.
Linux (ubuntu-22.04) and macOS (macos-14) are fully supported.
Windows (windows-2022) is best-effort with continue-on-error.
Crash artifacts are uploaded on failure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>