ci(bgfx): re-enable bgfx as a dedicated cached job

[apotema]

What

Re-enable the bgfx backend in CI as a dedicated, cached job. bgfx builds clean on Zig 0.16 now (zbgfx is on 0.12.0), and I verified locally on macOS that the backend tests pass and the standalone demo links + initializes the Metal renderer. The only reason it was gated off is the C++ compile cost — so this gives it its own job with actions/cache instead of inlining it into the matrix.

Changes (only .github/workflows/ci.yml)

• Removed the stale if: false step "bgfx backend tests + cross-compile compile-check" in build-and-test. Its comment blamed "zbgfx-0.10.1's addCSourceFiles 0.16 migration" — stale (now 0.12.0). The step also ran four -Dtarget=... cross-compile checks; those are dropped (cross-compiling bgfx's C++ to 4 targets would multiply an already-expensive build; the host macOS/Metal target is what's verified).
• Added job bgfx-build ("bgfx backend build + test (macOS)"):
  • runs-on: macos-latest — bgfx is verified there with Metal, and unlike raylib it has no hexops/xcode-frameworks transitive dep, so the macOS runner fetches + links cleanly.
  • Same sibling-checkout layout as build-and-test (labelle-assembler/ + labelle-gui/ for the flow-codegen path-dep), mlugg/setup-zig@v2 0.16.0.
  • actions/cache@v4 over ~/.cache/zig + ~/Library/Caches/zig + the bgfx .zig-cache dirs, keyed on hashFiles('.../backends/bgfx/build.zig.zon') with a bgfx-${{ runner.os }}- restore-key — so the compiled C++ persists between runs and a zbgfx bump busts the cache.
  • Runs cd backends/bgfx && zig build test then cd backends/bgfx/example && zig build.

Verification (macOS arm64, Zig 0.16.0)

• YAML parses (yaml.safe_load).
• backends/bgfx → zig build test: pass.
• backends/bgfx/example → zig build: pass (produces the Metal demo binary).

Note

The first CI run on this job will be slow (cold cache → full bgfx C++ compile, tens of minutes). Subsequent runs restore from the cache and are fast; a zbgfx version bump intentionally triggers one slow rebuild.

[gemini-code-assist]

Note

Gemini is unable to generate a review for this pull request due to the file types involved not being currently supported.

[cursor]

PR Summary

Low Risk
Workflow-only change; no application code, with the main caveat that the first cold-cache run will be slow and cache invalidation depends on the .zon hash.

Overview
bgfx is back in CI without slowing the main build-and-test matrix. The old if: false step (stale Zig 0.16 / zbgfx excuse plus four cross-target zig build test runs) is removed from build-and-test.

A new bgfx-build job on macos-latest mirrors the sibling labelle-gui checkout and Zig 0.16.0 setup, then actions/cache@v4 over global and project Zig caches keyed on backends/bgfx/build.zig.zon. It runs backends/bgfx tests and builds the Metal standalone example only on the host macOS target—no Linux/Windows cross-compile gate.

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