ui : reuse completed agentic turn sections during streaming
deriveAgenticSections runs in a $derived invalidated per streamed chunk,
but re-derived every turn of the session each time, so per-chunk cost grew
with session length. Cache completed turns keyed by their assistant message
plus reference checks on every field that feeds derivation; only the
streaming turn recomputes. Cache hits return the same section objects, so
tool block props stay stable and skip their per-chunk re-derive.
Assisted-by: pi:zai-org/GLM-5.3
