`_record_streamed_assistant_text` grew the turn's visible text with `+=`
on an attribute. Python only grows a string in place when the target is a
local variable, so this copied the whole text on every delta.
The loop runs once per streamed token, so a reply of length N costs about
N squared in copying. A 200 KB answer arriving in 4-character deltas
moves several billion characters and burns seconds of CPU in the loop the
file itself calls the hottest one in the agent.
The text is now kept as a list of pieces and joined when read. Reading
happens at turn end and on interrupt, not per delta, so the whole turn is
linear in the length of the reply. `_fire_stream_delta` used to join on
every token just to ask if the text was empty. That check now looks at
the parts list.
`_current_streamed_assistant_text` becomes a property over that list, so
the seven readers and the call sites that clear it between turns keep
working unchanged. Reading does not collapse the pieces, because a delta
landing between the join and the write back would be lost.
Measured with 8-character deltas: adding 20k deltas to an already long
text took 3.1 times as long as the first 20k before, and 0.9 times after.
Adolanium
7caee2898b