[spacedevin]

Motivation

screenshot.yml fires on every PR that touches crates/** (most of them) and, in parallel with the verify matrix, rebuilt its whole toolchain by hand: apt-get install of imagemagick/freetype/cmake/…, a libmgba 0.10.5 source build (cached, but only that one piece), dtolnay/rust-toolchain for the nightly, cargo install --locked agb-gbafix compiled from source every run, and setup-node. ci.yml's rom and verify jobs had already moved every one of those into ghcr.io/schlopai/chuggie-toolchain — the image's Dockerfile lists them as exactly the per-job cost it exists to remove — leaving this as the one job still paying the old way. It also builds the same overworld-demo ROM that verify overworld-demo builds in the same PR run.

Change

Of the two options (fold into ci.yml, or reuse the container), this is the smaller one: the job keeps its own workflow, trigger paths, name and artifact, and only the setup changes.

• container: is the same image and the same tag ci.yml pins (8f52ecdbceed), with the same credentials: block; a comment says to move the two together.
• permissions: { contents: read, packages: read } — packages: read is what the ghcr pull needs, as in ci.yml.
• Removed: the apt step, the libmgba cache + source build, the rust toolchain step, cargo install agb-gbafix, setup-node, and the explicit MGBA_PREFIX: /home/runner/mgba-prefix env. The image sets MGBA_PREFIX=/usr/local/mgba, which scripts/shot_common.sh's find_mgba_prefix reads first; ensure_gba_shot then compiles tools/gba-shot.c against it with the image's cc, and screenshot.sh finds the image's ImageMagick convert for the PPM→PNG step.
• Kept as-is: checkout, npm ci (with its comment about .cargo/config.toml's patch), the screenshot.sh invocation with TISH, the upload step, timeout-minutes: 40.

About the libmgba cache. The brief was to keep it; I removed it instead and want to flag that explicitly. In the container the library is prebuilt at /usr/local/mgba and nothing would ever populate or read ~/mgba-prefix, so the cache step would be dead weight alongside a Build libmgba step that never runs. The property the cache was providing — not rebuilding libmgba per run — is what the image provides now. If you would rather keep the fallback path for a runner without the image, say so and I will put it back as a conditional.

Verification

actionlint and a YAML parse are clean. Runtime check is the PR's own run of this workflow (it triggers on .github/workflows/screenshot.yml).

Not done here

verify overworld-demo in ci.yml already builds, boots and checks that this ROM draws something (scripts/verify_default.sh); the only thing this workflow adds is the uploaded PNG. Whether that is worth a separate ~2-minute job per PR is a product call, not made in this PR.