refactor(gui): split the AppState tests by what they exercise

state/tests.rs was 2169 lines: 54 tests on eleven areas of AppState, interleaved with the fixtures they share. Each area is its own module now — device_list, bindings, lighting, transient_identity, camera, profile_scope, device_names, reload, smartshift, asset_targets, wheel_resolution — and tests.rs keeps the imports and the fixtures more than one area uses (direct_inventory, app, lighting_changed, superseded_litra_light, the known-mouse state); a fixture only one area uses moved with it. Every child starts with `use super::*`, so the tests read as before; the two `super::` paths that meant the state module now go through the shell's imports.

Pure move: no test body changed; 54 tests before and after.