[keys-i]

What changed

• extended the existing Ubuntu, macOS, and Windows launcher step to test both local and streamed execution
• feeds checked-out run.sh to bash -s so it must download and unpack an archive
• pins that archive to the exact ${{ github.sha }} candidate being tested

Why

The existing check only exercised local checkout discovery. The streamed path could regress independently, and mutable branch URLs can pair a script with a different archive revision. This keeps both inputs on one candidate without another job, action, or dependency.

Validation

• local launcher self-test
• pinned streamed launcher self-test against merged main
• Prettier 3.9.6
• actionlint 1.7.12
• git diff --check

Progresses #9.