Store GUI machines as isolated TOML configurations

[iblowmymind]

iris-gui currently stores machine configurations inside gui.json, duplicating the emulator’s TOML format and sharing machine state paths. This change discovers and edits canonical machines/<name>/<name>.toml files alongside gui.json and runs each machine from its own directory.

• Keep GUI preferences in gui.json; save machine settings using the same TOML schema consumed by the emulator.
• Store all TOML filesystem paths relative to the machine directory. New disks default to disks/, with private NVRAM and NVEEPROM alongside the TOML.
• Migrate existing JSON machines with a preferences backup, preserve existing disk files, and isolate battery-backed state. Preserve malformed configurations and report failures.
• Support sandboxed macOS storage and bookmarks, isolate App Store CHD diffs, and guard working-directory changes while emulator operations are active.
• Remove TOML import/export actions and use the saved TOML directly for GUI and developer launch paths. Update configuration documentation.
• In a separate commit, change the About description to “Emulator for various SGI machines”.

Validation:

• cargo test -p iris-gui --locked --offline: 69 passed.
• cargo test -p iris-gui --features appstore,pcap --locked --offline: 73 passed.
• cargo check -p iris-gui --features bundled,pcap --locked --offline: passed.
• Tests cover migration, relative paths, machine discovery and lifecycle, private persistent state, external TOML edits, rename/delete retention, and sandbox resource handling. Headless lifecycle tests passed outside the tool sandbox after its shared-memory restriction prevented the initial run.
• Signed macOS sandbox GUI behavior, visual GUI interaction, full IRIX disk boot, and Linux AppImage runtime behavior remain untested.