The redraw path rebuilt the full transcript on every key event:
filtered the agent's full message slice, refreshed tool path maps,
walked every message through the per-message render cache, and
re-assembled the entire chat line buffer. With a long session, the
O(N) work per keystroke made typing visibly lag.
Add an idle render cache: the previously built chat is reused when
nothing relevant changed (terminal width, transcript revision,
status notes, help/update banners, expand-all). The agent now
exposes a cheap monotonically increasing Revision() that ticks
whenever messages are appended or replaced, so the cache key stays
trivial. Live turns (busy/streaming/tool-call mutations) keep the
old rebuild path.