## Why
The Python SDK must treat the runtime's initial goal turn and its
continuations as one logical operation. That requires a private
lifecycle engine before the public API can return the existing turn
handle and result types.
## What
- start goals by composing the existing clear/set goal RPCs
- enforce persisted, idle threads and a bounded startup handshake
- coalesce continuation notifications under a stable logical turn ID
- aggregate items, usage, timing, and terminal status
- support rollover-aware steering, interruption, cancellation, and
cleanup
- provide equivalent sync and async internals
This is the second PR in the stack and intentionally adds no public API.
## Test plan
- online CI, including the Python SDK suite
- behavioral coverage is added in the following two stack PRs
Ahmed Ibrahim