Make GitHub Release the clear install path

[rsasaki0109]

What changed

• Add a latest-release badge and keep the GitHub Release wheel as the supported no-source install path.
• Replace the stale source-only install wording with the versioned v0.4.1 wheel command.
• Add a short launch-discussion and star CTA after the LiDAR-camera evidence.
• Add a regression test that derives the wheel URL from the project version and rejects PyPI/source-only drift.

Why

The quickstart correctly installed from GitHub Releases, but the lower Install section still said Calibrex was distributed only from source. The landing page should present one consistent, testable path now that PyPI publishing is intentionally skipped.

User impact

Visitors get a visible release signal, one copyable stable-install command, and a clear place to share real calibration results or useful failures.

Validation

• README/public-dataset tests: 32 passed
• Focused release-install test: passed
• Ruff check and format check: passed
• MkDocs strict: passed
• git diff --check: passed