CI: validate website against the published release

[TheAmericanMaker]

What changed

• Resolve the latest published codecartographer-pi version and check out the matching CodeCartographer v<version> tag for website consistency tests.
• Add workflow_dispatch so CI can be run manually from the Actions tab.
• Update verification guidance to describe the published-version source boundary.

This makes the website check compare against the released version the site describes. Previously, the workflow checked out the repository's default branch, which can contain unreleased changes.

Verification

• npm view codecartographer-pi version returned 0.26.0.
• CODECARTO_UPSTREAM=$PAPERCLIP_RUN_SCRATCH_DIR/cc-v0.26.0 npm test returned 8 tests passed, 0 failed, against CodeCartographer tag v0.26.0.
• git diff --check passed.
• gh workflow list --all showed CI as active.
• On merge commit fde5ddb92a7187417851311e266613156f3a4e95, the Actions API returned only the pages build and deployment run (success, event dynamic); there is no CI push run.
• PR Update site version to v0.26.0 #24's merge commit has one parent (fd598e1), while its PR commit is 71bd39e, consistent with squash merge.

GitHub's run listing and active workflow state do not explain why the CI push event was not created. Cause remains unknown. The merge commit message has no [skip ci] marker. This PR's GitHub Actions result is pending.

Not run on Windows; CI uses Ubuntu.