Function handle_create_window 

pub(super) fn handle_create_window(
    state: &mut State,
    window_id: String,
    workspace_id: String,
) -> Vec<Event>

Phase E.5 — record a new window→workspace mapping. Validates the parent workspace exists; otherwise emits Event::Error (non-fatal). Idempotent on duplicate window_id: re-issuing for the same window updates the workspace pointer if it changed, or no-ops if identical.