P13: dice — crit/fumble animation + roll visibility

- Crit/fumble emphasis: RollTray (and the Dice page result) highlight a nat-20 /
  critical-success as "✦ Critical ✦" (gold pop + glow) and a nat-1 / critical-
  failure as "✦ Fumble ✦" (red shake). naturalD20() exported from notation.
- DM sees players' rolls: RollFeed now also renders on the Combat page (players'
  rolls already broadcast to the GM).
- Players see the DM's public rolls: new gmRoll message — a hosting GM's rolls
  (Dice page + rollAndShow) broadcast to the table as "GM".
- DM secret-roll toggle (rollStore.secret) on the Dice page suppresses the broadcast.
- Realtime e2e: GM public roll appears in the player's table feed.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/index.ts

@@ -100,6 +100,7 @@ export function buildServer() {
           case 'seatGrant': hub.seatGrant(sender, m.gmSecret, m.targetPlayerId, m.character); break;
           case 'playerPatch': hub.playerPatch(sender, m.characterId, m.diff); break;
           case 'playerRoll': hub.playerRoll(sender, m.characterId, m.label, m.expression, m.total, m.breakdown); break;
+          case 'gmRoll': hub.gmRoll(sender, m.gmSecret, m.label, m.expression, m.total, m.breakdown); break;
         }
       });
       socket.on('close', () => { clearInterval(refill); hub.disconnect(sender); });