Integration notes

• The SSE helpers are transport-focused and independent of event store persistence models, routing logic, or authorization policies.
• Event identifiers are preserved verbatim (no trimming) to support opaque schemes where whitespace may be meaningful.
• Validation occurs at construction time; once an EventId is created, it is guaranteed wire-safe.
• This crate still does not provide a convenience Actix responder or stream lifecycle abstraction for SSE endpoints.
• The Last-Event-ID header extraction follows the same single-header-or-error pattern used by the extract_idempotency_key function in the idempotency module.
• The heartbeat helper surface is intentionally wire-only: this crate exposes interval data and rendered frames, not timer orchestration.

For implementation guidance and internal conventions, see developers-guide.md.