The session's owning store (and its live in-process handle) is selected per
request by projectId. start() sent projectId but answer/resume/getSession did
not, so any project-scoped session broke on the first answer (a different store
resolved → session not found / no live handle). The client now captures the
start projectId and reuses it on every subsequent call. Closes the multi-project
session-identity residual.
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