broadcaster.ts: Write tests for message ordering guarantees

[Kingsman-99]

Description

src/broadcaster.ts delivers messages to subscribers, but there are no tests confirming that messages are delivered in the order they were broadcast. Out-of-order delivery can cause subtle bugs in state machines that depend on event sequencing.

Acceptance Criteria

• Test: three messages broadcast in sequence arrive at the subscriber in the same order
• Test: a subscriber added after some messages are broadcast does not receive missed messages
• Test: removing a subscriber mid-sequence stops delivery for subsequent messages only
• Unit tests pass

Context

• Target file: src/broadcaster.ts
• Tests should live in src/__tests__/broadcaster.test.ts