* fix(tui): delineate assistant responses from details
Add a muted Response marker before assistant text when thinking/tool details are visible so reasoning and final output do not visually run together.
* fix(tui): account for response separator height
Keep virtual transcript estimates aligned with the new response separator and avoid allocating trimmed copies of long assistant text.
* fix(tui): gate response separator estimate on details
Only add response-separator height when assistant details actually render, and use a non-allocating body-text check.
* fix(tui): skip empty detail height estimates
Do not add virtual transcript height for assistant details when no thinking or tool detail UI will render.
* fix(tui): estimate details by section visibility
Pass resolved thinking/tool visibility into virtual height estimates so hidden detail sections do not reserve response-separator rows.
