Adds a parameter-panel example (cargo run -p gpui_widgets --example controls)
that demonstrates every form control: float/rational/angle sliders with
keying diamonds, a spinbox, a combo box, checkboxes, a radio group, a color
picker and a curve editor, printing each edit as a request event. Marks all
W1 checklist items done in docs/zh/oak-app-rewrite.md. 65 tests pass.
