[ZhuchkaTriplesix]

Closes #1562.

• LocalExtensionInstaller.installArchive extracts into a hidden sibling folder, validates there, then swaps (old -> backup, new -> place, delete backup; restore on failure).
• HttpMarketplaceRepository.install uses the same path, so removed files no longer linger and common-root stripping matches the local installer.
• The registry skips dot-prefixed folders, so a staging/backup folder left by a crash is never loaded as an extension.
• Tests: failed upgrade keeps v1; upgrade replaces files instead of mixing versions.

[ZhuchkaTriplesix]

🔍 Post-Merge Code Review & Verification

Significant reliability upgrade for the extension ecosystem:

1. Atomic Upgrade Pipeline (local_extension_installer.dart):

   • The two-phase extract-to-staging (.installing) and atomic rename swap (.backup -> target) guarantees that broken archives or extraction failures cannot destroy an active, working extension.
   • Crash resilience: leftover .backup directories are automatically recovered or cleaned up on the subsequent installation cycle.

2. Registry Hygiene (local_extension_registry.dart):

   • Filtering out dot-prefixed directories prevents half-installed or backup artifacts from appearing as ghost extensions in the UI.

3. Shared Invariant with Marketplace:

   • Routing HttpMarketplaceRepository.install through installArchive eliminates divergent installation code paths.