Refactor/cargo workspace

[cyb0rg56]

The app is now a four-part Cargo workspace like fan-control's. It builds, and cargo fmt --check and cargo clippy --workspace --all-targets are clean. All 121 tests pass (69 in data, 52 in hardware). I haven't run the app itself, so the GUI smoke test from the plan is still to do.

Crate	Contents
hardware/	ccd and ddc. The DDC test helpers are now in ddc::test_util, available through a test-util feature so data's tests can use them.
data/	Config, persistence, profiles, startup, hotkey manager, tray, and the hardware job queue. The new presentation.rs holds the monitor view models, and the new state.rs holds AppState.
ui/	A thin AppModel wrapper, the views, effects.rs, subscriptions.rs and keys.rs.
root	main.rs calls ui::run(...). build.rs and packaging are unchanged.

How the split works:

• State: AppState::update(Input) returns a list of Effect's. It never creates a cosmic Task.
• Effects: there are five kinds:
  • Spawn(Job): hardware work, config ops or profile listing.
  • Delay: the slider debounce.
  • MonitorsDetected: the UI rebuilds the navigation bar.
  • ProfilesUpdated: the UI refreshes the tray menu.
  • ShowHotkeys: the UI switches to the Hotkeys page.
• UI: the UI turns these into tasks. Message is now Data(Input) plus the UI-only messages (window, tray, settings drawer, clipboard, URLs). Views read state only through getters.
• Hotkeys and tray: data::hotkeys and data::tray no longer use cosmic. The UI wraps them as subscriptions.
• Dependencies: data doesn't depend on libcosmic, and hardware doesn't depend on cosmic, tray-icon or global-hotkey (checked with cargo tree).