feat(lotus): io.lotus.set_screenshare for the host call bar (cinny #43)

toWidget `io.lotus.set_screenshare { on?: boolean }` (omit to toggle)
starts/stops sharing via the view model instead of the host clicking EC's
hidden button. The host sends it with Capability Delegation
(`delegate: "display-capture"`) where supported, so getDisplayMedia keeps
the user's click on engines that require it. `controls_state` now carries
`screenshareAction: true` so the host only uses it on a fork that has it.

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

src/lotus/lotusActions.test.ts

@@ -30,6 +30,7 @@ describe("LotusWidgetActions", () => {
       LotusWidgetActions.SetLayout,
       LotusWidgetActions.OpenSettings,
       LotusWidgetActions.ToggleReactions,
+      LotusWidgetActions.SetScreenshare,
     ];
 
     expect(new Set(LOTUS_TO_WIDGET_ACTIONS)).toEqual(new Set(expectedToWidget));