v6.5.1 — discover existing Unity editors on native runners

Fixed

• Unity preflight discovers existing editors before installing one, on Windows
  and macOS runners. Found on the first real self-hosted macOS run.

  • The bug: v6.5.0 treated "not in the runner account's Unity CLI
    registry" as "not installed". That registry is per OS account, so editors
    an admin installed with Unity Hub were invisible. The run reported the
    editor MISSING, then failed on the unwritable /Applications/Unity/Hub/Editor.
    The native lanes used to find those editors by path; v6.5.0 had dropped
    that lookup.

  • The fix: preflight now searches, in order:

    1. UNITY_EDITOR;
    2. the CLI registry;
    3. UNITY_PREFLIGHT_INSTALL_ROOT;
    4. the runner-managed fallback root;
    5. the CLI's install path;
    6. the standard Unity Hub and legacy locations the lanes always used.

    A match is registered with unity editors add, so the CLI confirms its
    exact version; a mismatched editor is never used. Only then does it install.

  • Explicit UNITY_EDITOR: an override unknown to the CLI is now
    registered and version-checked, instead of being trusted unverified.

  • Fallback install root in CI: a missing editor whose configured install
    path isn't writable goes to ~/Unity/Editors (new
    --fallback-install-root) instead of failing.

  • Module installs: before adding a module to an existing editor,
    preflight checks that the editor's own folder is writable, not the install
    root's.

  See docs/UNITY_PREFLIGHT.md § Editor discovery.