feat(installer): replace init with version-aware router, preserve legacy

Rename the old init script to init-legacy (unchanged) and introduce a
thin version-aware router as the new init entry point.

- Versions >= 0.31.0 (first cargo-dist release) and 'latest' are
  routed to the cargo-dist installer on GitHub Releases.
- Older versions are delegated to init-legacy, preserving full
  backward compatibility including the sh -s -- VERSION interface.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: fischeti