Serialize node installation beyond tool-handler completion

Sequential tool handlers do not guarantee sequential workflow transitions when result callbacks schedule node installation independently. Identify the full boundary: handler return, context-result callback, node pre-actions, tool registration and context installation. A second accepted user turn can invoke old-node tools while the first node is still being installed, causing conflicting actions to complete out of order. Reproduce this with event-held pre-actions rather than timing delays. A transition lock can restore installation order while still allowing an incorrect earlier handler decision to close the conversation; treat ordering and semantic argument validation as separate requirements. Do not claim that a per-response parallel-tool flag covers separate user turns.

Applies when: asynchronous result callbacks install conversation nodes after tool handlers return. Keywords: tool handlers callback node transition serialization preactions stale context