Refine embedded host SDK surfaces

[roackb2]

What changed

• add engine.sessions.readExisting(id) for direct persisted-session lookup without materializing the fallback session
• export HeddleEventType and HeddleEventTypeValue from the curated package entrypoint
• document both embedded-host APIs in the programmatic conversation-engine guide

Why

Embedded hosts currently need to scan listExisting() to safely reuse a deterministic session ID. Calling read(id) is not equivalent because it materializes Heddle's fallback session in an empty repository.

Hosts translating conversation activity into their own event stream also had to duplicate event-name string literals even though Heddle already owns a canonical vocabulary.

These additions expose the existing domain semantics directly without adding host-side workarounds or a parallel event model.

Impact

• persisted session lookup is direct and non-materializing
• host event adapters can compare against the canonical HeddleEventType values
• existing session and event behavior remains unchanged

Validation

• yarn lint
• yarn test — 613 unit tests and 285 integration tests passed
• yarn build
• packed and installed the unpublished package into an external embedded host
• external host typecheck, tests, and production build passed
• real three-turn direct and HTTP/SSE flows reused one durable session, produced validated artifacts, and exposed zero memory-tool events when using toolProfile.memoryMode: 'none'