When a turn is interrupted at the wrong moment, the append-only JSONL can
end up with a user message between an assistant tool_calls block and its
tool result. Providers reject this ordering (Anthropic 400/2013, OpenAI
'tool_call_ids did not have response messages'), making the session
permanently unrecoverable from the UI.
_repair_tool_pairing() runs in ConversationStore.load() and:
- moves a real tool result found later in the thread to sit right after its call
- synthesises a placeholder result for a call with no matching tool message
- is idempotent — well-formed threads pass through unchanged
Fixes#331
