feat(lotus): layout, settings and reactions over the widget API (cinny #43)

The Lotus host drove these by clicking EC's hidden controls in our DOM and
read screenshare/layout state from it, which only works while the frame is
same-origin and breaks whenever a data-testid changes. New actions:

- toWidget io.lotus.set_layout { layout: "grid" | "spotlight" } → the layout
  switch view model's setLayout.
- toWidget io.lotus.open_settings { open?: boolean } → setSettingsOpen
  (toggle when `open` is omitted).
- toWidget io.lotus.toggle_reactions → a window event the reactions button
  listens for.
- fromWidget io.lotus.controls_state { screensharing, layout } on change; its
  arrival tells the host the fork supports the actions above.

Screensharing stays host-driven for now: getDisplayMedia needs the user's
click to reach this frame (Capability Delegation), which a widget message
doesn't carry.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/lotus/lotusActions.test.ts

@@ -27,6 +27,9 @@ describe("LotusWidgetActions", () => {
       LotusWidgetActions.Decorations,
       LotusWidgetActions.SetDeafen,
       LotusWidgetActions.SetAudioOutput,
+      LotusWidgetActions.SetLayout,
+      LotusWidgetActions.OpenSettings,
+      LotusWidgetActions.ToggleReactions,
     ];
 
     expect(new Set(LOTUS_TO_WIDGET_ACTIONS)).toEqual(new Set(expectedToWidget));
@@ -44,5 +47,8 @@ describe("LotusWidgetActions", () => {
     expect(LOTUS_TO_WIDGET_ACTIONS).not.toContain(
       LotusWidgetActions.DenoiseState,
     );
+    expect(LOTUS_TO_WIDGET_ACTIONS).not.toContain(
+      LotusWidgetActions.ControlsState,
+    );
   });
 });