zebra-spaces: drop tiles of demoted-to-listener peers immediately

Was: when a speaker was bounced to listener their tiles froze on every
other peer until either the SFU's removePublisher path finally fired
'ended' on the subscriber's tracks or the 60s/120s mute window reaped
them. Pion's OnConnectionStateChange can lag and SSE renegotiation can
race, leaving visibly-frozen tiles for tens of seconds.

Authoritative removal: when role-change strips speak rights from
peer X (canSpeak(prev) && !canSpeak(next)), drop every kind of X's
video tile on the receiver side immediately. The demoted user can't
publish anymore by definition, so the tiles are guaranteed-stale —
no need to wait for the data-plane path to catch up.

Mirrors the existing peer-left handler which removes by pubkey.

web/zebra-spaces.html

@@ -3191,6 +3191,21 @@ async function handleSignal(raw){
             if (canSpeak(myRole) && canSpeak(m.role) && !peers.has(m.uuid)){
               connectToPeer(m.uuid, myUUID < m.uuid);
             }
+            /* If the role transition stripped them of the ability to
+             * publish (anything -> listener), their camera/screen/game
+             * tiles can't get fresh RTP anymore. The demoted page calls
+             * sfuUnpublish* on its own side which should propagate via
+             * SFU 'ended', but that path is unreliable (Pion may not
+             * fire OnConnectionStateChange immediately and SSE renego
+             * can race). Authoritative removal: drop tiles by pubkey
+             * here, the moment the role change broadcast lands. */
+            if (canSpeak(prev) && !canSpeak(m.role) && subjPub){
+              try {
+                removeScreenTile(subjPub);
+                removeCameraTile(subjPub);
+                removeVideoTile('gameshare', subjPub);
+              } catch(_){}
+            }
           }
           renderRoom();
           reorderTiles();
@@ -3974,8 +3989,8 @@ logLine('', 'ready — pick a handle, type a rendezvous code, enter the space');
 
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