Demo: follower desync on rapid interaction / cold start

[rlyshw]

Follower iframe can get out of sync with source when:

1. Clicking before follower iframe finishes loading (ops lost)
2. Rapid clicking causes race conditions between batches

Fix: Add readiness handshake — follower sends {type:"ready"} to parent after player is bootstrapped. Parent buffers ops until follower signals ready.

Affects all demos but most visible in dice (wrong face values on follower).