ci: fan out test jobs from a shared build and move actions to Node 24

[LinuxJedi]

CI restructure

The serial Build, test, lint job is replaced by a fan-out:

• build compiles the release binary plus all test binaries once, then stages the finished binaries (main binary + unit-test, probe_golden, and image_regression executables, located via cargo test --no-run --message-format=json since their filenames are hash-suffixed) into a ~50 MB workflow artifact.
• unit-tests, probe-golden, and diagrom-smoke run in parallel as needs: build jobs that just download the artifact and execute the binaries directly — no toolchain, no rust-cache, no compile. Test executables are plain libtest binaries, so they accept the usual filters and --ignored/--nocapture flags as-is. The integration harnesses spawn the emulator through the compile-time CARGO_BIN_EXE_copperline path, which is the same absolute path on every hosted runner for this repo, so those jobs copy the binary into target/release first.
• m68k-core also fans out from build (fail-fast gating only — the vendored crate builds standalone into its own target dir, now cached via workspaces: crates/m68k; previously both m68k jobs cached the root target directory they never built, so the vendored core recompiled cold on every run).
• lint (rustfmt + clippy) runs alongside the build instead of gating it, since it needs no release binary.
• wasm, m68k-singlestep, docs, and gdb-remote stay independent as before.

Binaries are handed over as an artifact rather than through a shared cargo cache deliberately: an earlier revision of this branch shared the build tree via a rust-cache shared key, and although the fan-out jobs restored it with a full key match, cargo still recompiled the copperline and vendored copperline-m68k crates in every job — local path crates are fingerprinted by source mtime, and a fresh checkout is always newer than the cached artifacts. The build job keeps a plain rust-cache for cross-run dependency caching.

DiagROM hardening (from review): the extracted ROM's sha256 is now verified on every run, covering the cache-hit path that previously skipped the (zip-only) checksum.

Notes:

• The unit-test split (--lib --bins equivalent) loses nothing: the crate has zero doctests, and the build job's cargo test --no-run still compile-checks the asset-gated integration tests.
• No required status checks reference the old job names, so the renames break nothing.

Node 20 deprecation fix

All first-party actions move off the deprecated Node 20 runtime across every workflow: checkout v4 -> v7, upload-artifact v4 -> v7, download-artifact v8, setup-node v4 -> v7, cache v4 -> v6. Each was verified to declare runs.using: node24 at its current major, and the release notes across the jumps contain no breaking changes for this usage. The MyST docs jobs' own node-version moves 20 -> 24 as well, since Node 20 itself is EOL.

[Copilot]

Pull request overview

Restructures GitHub Actions CI to reduce wall time by fanning test jobs out from a shared build cache, and updates first-party GitHub Actions to versions that run on the newer Node runtime (plus bumps docs workflows to Node 24).

Changes:

• Replace the monolithic “Build, test, lint” CI job with a cached “build” job and parallel needs: build test jobs (unit-tests, probe-golden, m68k-core, diagrom-smoke), while keeping other jobs independent.
• Update workflow actions (e.g., actions/checkout, actions/upload-artifact, actions/setup-node, actions/cache) to newer majors and set MyST docs workflows’ Node version to 24.
• Improve caching for the vendored crates/m68k jobs by targeting that workspace’s target directory.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
.github/workflows/ci.yml	Fans out CI test jobs from a shared build + rust-cache key; updates several actions and Node versions; refactors DiagROM smoke test to use caching and artifacts.
.github/workflows/windows.yml	Bumps checkout/upload-artifact action majors for Windows packaging workflow.
.github/workflows/macos.yml	Bumps checkout/upload-artifact action majors for macOS packaging workflow.
.github/workflows/appimage.yml	Bumps checkout/upload-artifact action majors for AppImage packaging workflow.
.github/workflows/wasm-demo.yml	Bumps checkout action majors for wasm demo publish workflow (including second checkout for site repo).
.github/workflows/docs-site.yml	Bumps checkout/setup-node action majors and Node version for docs-site publish workflow.
.github/workflows/docs-release.yml	Bumps checkout/setup-node/upload-artifact action majors and Node version for docs PDF release workflow.
.github/workflows/flatpak.yml	Bumps checkout action majors for Flatpak CI workflow.
.github/workflows/locks.yml	Bumps checkout action major for lockfile verification workflow.
.github/workflows/linux-present.yml	Bumps checkout action major for Linux present smoke workflow.
.github/workflows/homebrew.yml	Bumps checkout action major for Homebrew style/audit workflow.

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