[Chemaclass]

Background

Documentation checks shared one concurrency group across all PRs, so queued builds cancelled checks on unrelated PRs. The Windows installer check also rejected a plain-text mock download as non-executable.

Changes

• Give each ref a separate docs-check group and preserve main runs.
• Use a shell script for the installer download fixture, keeping exact-content and permission checks.
• Validate all four installer outcomes on macOS and Linux strict mode, plus formatting and diff review.

Checklist

• I updated the CHANGELOG.md to reflect the new feature or fix (no public behavior change)
• I updated the documentation to reflect the changes (CI and test fixtures only)

[Chemaclass]

[Codex] These fixes are included in #1446 and propagated through the remaining stack. Closing this duplicate so validation runs on the branches that will merge.