feat(lotus): work when served from its own origin (cinny #43)

Two things tied EC to the host's origin:

- Widget message check: matrix-widget-api's strictOriginCheck compares
  ev.origin with THIS frame's origin, so on call.chat.lotusguild.org every
  message from chat.lotusguild.org would be dropped and calls would not
  start. restrictToHost() instead requires ev.source === window.parent and
  ev.origin === parentUrl's origin. Same-origin deployments keep working
  (the host origin is our own origin there), and it is stricter than
  before: the sender must also be our parent window.
- Soundboard: the host's blob: clip URL is origin-bound. io.lotus.inject_audio
  now accepts the clip's bytes (`audio`, ArrayBuffer, <= 8 MiB) and prefers
  them over `url`; hosts that only send `url` are unchanged.

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

src/widget.ts

@@ -24,6 +24,7 @@ import { Config } from "./config/Config";
 import { seedSettingsFromConfig } from "./settings/settings";
 import { ElementCallReactionEventType } from "./reactions";
 import { LOTUS_TO_WIDGET_ACTIONS } from "./lotus/lotusActions";
+import { restrictToHost } from "./lotus/lotusWidgetOrigin";
 
 // Subset of the actions in element-web
 export enum ElementWidgetActions {
@@ -94,16 +95,11 @@ export const initializeWidget = (
       const parentOrigin = new URL(parentUrl).origin;
       logger.info("Widget API is available");
       const api = new WidgetApi(widgetId, parentOrigin);
-      // [lotus] matrix-widget-api's PostmessageTransport defaults
-      // strictOriginCheck to false, which would let any frame holding a
-      // handle to our window post toWidget actions (including the
-      // io.lotus.* actions below). The Lotus deployment serves EC
-      // same-origin with the host (cinny loads /public/element-call/index.html),
-      // so globalThis.origin === parentOrigin and this check passes safely.
-      // A cross-origin deployment would need to compare ev.origin to
-      // parentOrigin instead, since strictOriginCheck compares against
-      // globalThis.origin.
-      api.transport.strictOriginCheck = true;
+      // [lotus] Only messages from the host (our parent, at parentUrl's
+      // origin) reach the widget API. matrix-widget-api's own
+      // strictOriginCheck compares against THIS frame's origin, so it only
+      // worked while EC was served from the host's origin (cinny #43).
+      restrictToHost(api.transport, parentOrigin);
       api.requestCapability(MatrixCapabilities.AlwaysOnScreen);
       api.requestCapability(MatrixCapabilities.MSC4039DownloadFile);