fixo::SceneEditorWidget

Studio-side editor for a preset's `scenes` array.

Detailed description

Per-preset Studio tab. Operates on a working copy of a Preset: - left column: list of scenes with add / remove / rename buttons - middle: per-channel dropdown of stanza ids for the selected scene - bottom: live validation indicator (valid / claim conflict)

The widget owns its working copy. Callers seed it via setPreset() and read back the edited form via preset() once the user clicks Save (or whatever the host activity wires the commit to). The widget never writes to a SyncSection itself — Studio's existing preset-save flow is unchanged.

Validation matches what `Runtime::activate` and `Preset::fromJson` would reject: every assignment names a known channel, the assigned stanza belongs to that channel, and the combined claim across all assignments is conflict-free.