Add GPT autodiscovery advisory to status output

[GlitchSlayed]

Motivation

• Make rustyuki status aware of Discoverable Partitions Specification (DPS) so users get a helpful advisory when their root partition already carries a discoverable GPT type GUID.
• Avoid silently baking redundant or fragile root= cmdline tokens into UKIs by providing advisory (not automatic) guidance to remove them when GPT autodiscovery will work.

Description

• Implemented discoverable_root_advisory in src/app.rs which reads the configured cmdline file, checks for a root= token, reads /proc/mounts to find the device mounted at /, and queries lsblk -no PARTTYPE for that device to obtain the partition GPT type GUID.
• Added dps_root_guid mapping for architecture-specific DPS root GUIDs and parse_root_mount_source helper to extract / mount source.
• Added discoverable_root_message to format the two advisory outputs: an [INFO] advisory when root= is present and the partition GUID matches DPS, and an [OK] confirmation when root= is absent and the GUID matches; the code emits no automatic changes to the cmdline file.
• Updated tests and added unit coverage for root mount parsing, DPS GUID lookup, and both advisory message paths, and adjusted the status integration test to use a temporary cmdline file; changes are all in src/app.rs and test updates in tests/integration_cli.rs.

Testing

• Ran formatting with cargo fmt --all which completed successfully.
• Ran the full test suite with cargo test which completed successfully; all unit and integration tests passed.
• Existing status/integration tests were updated to exercise the new advisory behavior and the new unit tests cover the parsing and message logic.

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Codex Task