Retain shared Candidate images during uninstall

[BrettKinny]

What changed

• remove only the install-owned image alias during uninstall
• retain published Candidate digest/tag references as shared runtime cache
• keep Bash and native PowerShell lifecycle behavior aligned
• add a regression test where the shared release image cannot be removed because another Box uses it

Root cause

The uninstallers treated both the install alias and the globally shared immutable Candidate reference as resources owned by one Install identity. Purging a second Box therefore failed after removing its container when another Box still used the same image.

Validation

• all executable tests/test-*.sh tests pass
• lifecycle ownership regression fails before the fix and passes after it
• isolated Unraid replay against v1.2.1-rc5: second Box purged successfully, shared digest retained, existing squarebox container unchanged