Refactor chat turn service boundaries

[roackb2]

Summary

• restructure chat turn runtime code into class-owned subdomains for context, runtime, preflight, persistence, memory, host, and trace
• remove direct runConversationTurn exports so callers use EngineConversationTurnService / createConversationEngine(...).turns
• group compaction/runtime/session inputs to reduce repeated field reassignment and keep session persistence owned by sessions

Validation

• yarn typecheck
• yarn test src/tests/unit/core/chat-turn-runtime.test.ts src/tests/unit/core/chat-turn-persistence.test.ts src/tests/unit/core/chat-turn-memory-maintenance.test.ts src/tests/unit/core/chat-turn-host-bridge.test.ts src/tests/unit/core/conversation-engine.test.ts src/tests/integration/chat/chat-runtime.test.ts
• yarn lint