Platform management: detection, automatic downloads and safe removal - #875

Changes

• Fixes Installed Title and Sorting: Read and cache SDK build identifiers from installed Xcodes. Match installed runtimes by architecture as well as build, and reload the Platforms window when the downloadable catalog changes.
• Add optional platform selection in Downloads settings. Download one matching runtime per selected platform alongside an Xcode download, skipping installed/installing runtimes. The default selects none; this uses the existing runtime installer and its active-Xcode requirements.
• Add Uninstall to installed platform rows and offer Uninstall with Platforms only for installed runtimes that no other installed Xcode uses.

Reviewer checks

1. Inspect platforms for an installed Xcode using the Apple data source; check installed variants appear once.
2. Select platforms in Downloads settings and install an Xcode with matching SDK metadata.
3. Compare uninstall choices for Xcodes with shared versus exclusive runtimes; simulate an Xcode-removal failure and confirm its platforms are retained.

Consolidates #862, #868 and #870. Related independent topics: UI update and Usability improvements. The combined preview branch contains the complete app and tested resolutions for the shared code.

Related review topics: #873, #874.

Related issues

• Beta 6 has wrong looking platforms #844 — uses SDK builds from the installed Xcode bundle when release metadata is unavailable or does not describe a renamed beta.
• Broken Installed platforms #819 — matches installed simulator runtimes by build and architecture, preventing non-installed variants from appearing as deletable.
• Automatically download optional simulators #298 — adds optional automatic downloads for the platforms selected in Settings.
• Installed platforms versions incorrect #803 and Platforms list doesn't auto-update or show progress for deletion #668 — related installed-platform display and refresh reports that should be retested against this branch before closure.
• Auto set active and download SDKs #719 — covers selected runtime downloads once an Xcode install begins; it does not schedule Xcode installs or make the new Xcode active automatically.