feat(replay): Replay-Bypass guard — skip external side-effects on a deliberate replay (ADR-0027) (#10)

Backward-compatible capability upgrade: ReceivedMessage gains an additive Headers map; an optional HeaderPublisher interface (transports MAY implement) carries out-of-band headers; InMemoryTransport implements it. Redrive's new Bypass option stamps a bq-replay-bypass header; the runtime surfaces it to the handler's context; IsReplay + BypassExternalEffects let a handler re-run its idempotent core but skip effects that already fired. No envelope change (GR-1), no breaking Transport change. Concrete broker transports + other SDKs to follow.

Author: muhammetsafak