feat: mature hosted run lifecycle

[roackb2]

Summary

• add typed conflict, not-found, replay-window, and cancellation errors to the existing ConversationRunService
• let hosted turn runs await a host-owned result projection before publishing their terminal result
• expose retained run handles with their host-defined address so hosts can authorize without a duplicate registry
• make cancellation win late completion for startTurn / startContinue while preserving existing generic control-plane stop semantics
• refactor the hosted SDK example to delete its parallel context map and demonstrate safe terminal projection

Why

The hosted example and downstream integrations had to rebuild run retention, ownership lookup, and terminal projection around Heddle. These are run-domain responsibilities that belong in the existing coordinator. Product identity, authorization decisions, persistence contents, HTTP, and UI remain host-owned.

Verification

• yarn test:unit — 646 passed
• yarn test:integration — 291 passed
• yarn typecheck
• yarn eslint ...
• yarn build

This is PR 1 of a planned stack. The next PR will add transport-focused HTTP/SSE primitives and refactor the same example to use them.