fix(kun): preserve partial response on interrupt

[musnows]

Summary

• persist streamed assistant reasoning/text before an interrupted foreground turn is marked aborted
• keep the current stop button behavior as an interrupt so generated content remains visible after runtime recovery
• add a regression test for preserving partial assistant text after interrupt

Root cause

Assistant text deltas were emitted over SSE during streaming, but the accumulated assistant item was only persisted after a normal model completion. If the turn was aborted first, recovery from the runtime snapshot had no assistant item to render, so the UI appeared to roll back to the last user prompt.

Validation

• npm --prefix kun test -- loop.test.ts -t "keeps partial assistant text when interrupting a foreground turn"
• npm --prefix kun run typecheck

Notes

• Full npm --prefix kun test -- loop.test.ts still has 3 existing compaction assertion failures unrelated to this change.