CI: Build examples in parallel on pull requests

[krodak]

Overview

build-examples builds every example sequentially in a single job and takes ~57 minutes. Each example is its own SwiftPM package that recompiles JavaScriptKit and swift-syntax from scratch with no sharing between them, so the cost is dominated by repeating that swift-syntax compile back-to-back.

Two improvements, in two commits:

1. Build examples in parallel on pull requests. A matrix build-example runs one job per example concurrently, with an aggregate build-examples job gating on the matrix so the existing required status check of that name keeps working unchanged. main keeps the single build-examples-deploy job (full release build) plus the GitHub Pages deploy, so published artifacts are identical. Examples are built in release to match the deploy path, and each matrix step runs build.sh from the example directory via working-directory (mirroring Utilities/build-examples.sh).

2. Cache each example's .build. Keyed on the workflow file and the example's Package.resolved. On a warm cache the matrix jobs skip recompiling swift-syntax, which is the bulk of a cold example build.

Timing comparison (per-example matrix wall-clock)

setup	per example	examples wall-clock
today (single sequential job)	—	~57 min
parallel, no cache	~10 min	~10 min
parallel, cold cache (miss + save)	~10-12 min	~12 min
parallel, warm cache (hit)	~1-2 min	~2 min

Typical PRs change source, not Package.resolved or the workflow, so they hit the warm path: ~57 min → ~2 min. Cold runs (first run, or when deps/workflow change) pay a small cache-save overhead. GitHub's 10 GB cache budget is shared across the six example .build caches, so heavy eviction would fall back to cold runs.

No measurements were taken on the main deploy job; it is unchanged.

Question

I kept the required status check working by adding an aggregate build-examples job that depends on the matrix, so the required context name is unchanged and branch protection needs no edits. The alternative would be to drop the aggregate and make the per-example build-example (<name>) checks required instead. I assumed keeping the current build-examples name and required-check setup as-is is preferable - happy to switch to per-example required checks if you'd rather.