vellumwidget 0.3.0

• Adopted vellum's renamed vl_* graphics primitives (grid collision fix).

• Shiny input read-back. A widget rendered with vellumwidgetOutput() now reports the
  user's interactions to the server as reactive inputs keyed by the output id:
  input$<id>_selected (selected keys, state), input$<id>_click (list(key=),
  event), input$<id>_hover (hovered key or NULL, state), and
  input$<id>_brush (list(keys=, x0=, y0=, x1=, y1=), event). Values are element
  data keys, so they map straight back to your data. Emitted only in a live Shiny
  session — a static render is unchanged and produces no input traffic. See the new
  Using vellumwidget in Shiny article. (Driving the widget from the server — a
  vellumwidget_proxy() — is a planned follow-up.)

• Accessibility (a11y = TRUE, on by default). The interactive widget is now
  keyboard- and screen-reader-navigable, not a mute image:

  • the SVG is announced as an interactive chart (role="graphics-document" +
    aria-roledescription), labelled from the scene's title/description (which
    vellumplot sets automatically) or an explicit as_widget(alt =);
  • every mark is a focusable graphics-symbol with a roving tabindex — arrow
    keys move between marks, Enter/Space toggles selection, Escape exits;
  • a polite aria-live region announces the focused / selected mark;
  • a visually-hidden data table lists every mark for assistive tech.
    All gated on a11y; a11y = FALSE restores the previous output exactly. See the
    vellumplot Accessibility article.