feat: support external macOS installs

[llawliet12138]

Adds support for targeting macOS installations on external APFS drives.

Changes

• Detect mounted APFS System volumes dynamically
• List only System volumes, not Data volumes
• Automatically match the paired Data volume through APFS Volume Group ID
• Add external-drive validation with --require-external
• Add reversible interactive menus with Back navigation
• Add a final review page before any changes are written
• Add --demo mode for safe UI preview without disk, account, file, or reboot operations
• Add regression tests for volume targeting and menu behavior
• Update README usage for the external-drive workflow
• Write hosts and ConfigurationProfiles markers to SSV-aware Data volume paths

Tested

• Add regression tests for volume targeting, Data-volume writes, and menu behavior
• Successfully tested on macOS Tahoe 26.5.1 with an external APFS System/Data volume pair