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, so the cost is dominated by repeating that swift-syntax compile back-to-back.

This turns the pull-request build-examples job into a matrix - one job per example, running concurrently - so the wall-clock time drops to that of the slowest single example. 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).

Timing

setup	examples wall-clock
today (single sequential job)	~57 min
parallel matrix	~10 min

Each example still takes ~10 min on its own (dominated by compiling swift-syntax from scratch), but they now run in parallel. The main deploy job is unchanged.

Branch protection: required checks need updating

Because build-examples is now a matrix, the single build-examples status check is replaced by one per example. An admin needs to update the branch protection required checks for main: drop build-examples and add the six matrix contexts:

• build-examples (ActorOnWebWorker)
• build-examples (Basic)
• build-examples (Embedded)
• build-examples (Multithreading)
• build-examples (OffscrenCanvas)
• build-examples (PlayBridgeJS)

Use the job + matrix name exactly as above. The Run unit tests / prefix and the (pull_request) suffix shown in the PR merge box are display-only and are not part of the required-context string. Until this is done, the old build-examples context sits in "Expected — Waiting for status to be reported" and blocks merge.