fix(call): serve RTC transports to Element Call over MSC4515 — calls work again

Since the upstream v0.25.0 merge, Element Call in widget mode no longer
reads .well-known for its LiveKit transport; it asks the HOST via the
widget API (org.matrix.msc4515.get_rtc_transports, capability
org.matrix.msc4515.rtc_transports). cinny never granted the capability
nor implemented WidgetDriver.getRtcTransports(), so discovery returned
nothing and every join failed with "Call is not supported"
(MISSING_MATRIX_RTC_TRANSPORT) — the [LocalMembership] Multiple
Transport Errors line in the browser console.

- matrix-widget-api 1.17.0 -> 1.18.0 (adds MSC4515; also changes the
  sendDelayedEvent driver signature, adapted — parent delay ids were
  removed from the draft).
- Grant MSC4515RtcTransports in getCallCapabilities.
- CallWidgetDriver.getRtcTransports(): homeserver /rtc/transports
  (MSC4143) first, then the .well-known org.matrix.msc4143.rtc_foci list
  (what matrix.lotusguild.org advertises today). Unit-tested.

Server side needs no change: the well-known already carries the livekit
focus and the JWT service answers.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

package-lock.json

@@ -54,7 +54,7 @@
         "linkify-react": "4.3.3",
         "linkifyjs": "4.3.3",
         "matrix-js-sdk": "41.7.0",
-        "matrix-widget-api": "1.17.0",
+        "matrix-widget-api": "1.18.0",
         "millify": "6.1.0",
         "pdfjs-dist": "6.3.289",
         "prismjs": "1.30.0",
@@ -10045,9 +10045,9 @@
       }
     },
     "node_modules/matrix-widget-api": {
-      "version": "1.17.0",
-      "resolved": "https://registry.npmjs.org/matrix-widget-api/-/matrix-widget-api-1.17.0.tgz",
-      "integrity": "sha512-5FHoo3iEP3Bdlv5jsYPWOqj+pGdFQNLWnJLiB0V7Ygne7bb+Gsj3ibyFyHWC6BVw+Z+tSW4ljHpO17I9TwStwQ==",
+      "version": "1.18.0",
+      "resolved": "https://registry.npmjs.org/matrix-widget-api/-/matrix-widget-api-1.18.0.tgz",
+      "integrity": "sha512-4T2f2koWmx05p1BLcT/9YGGGPSXpPT+PA4Oap/5fjhXsWPxMGJiGL97YpANMYLKsnE1sScYFeuyxIgdc1Qo+Ew==",
       "license": "Apache-2.0",
       "dependencies": {
         "@types/events": "^3.0.0",