- spill-policy reserves the spill notice's byte cost inside maxInlineBytes, so
the replacement (preview + notice) never exceeds the documented model-facing
cap. When the notice alone fills the budget the preview is empty; when even a
notice-only replacement is not smaller than the original, the inline result is
kept (spilling would only add bytes).
- retention TextRetainer trims an oversized single suffix chunk to the last
suffixCap bytes on push, so tail/headTail retention stays bounded by suffixCap
instead of retaining and re-copying the whole chunk in finish() — this is the
spill preview path, which pushes the whole result as one chunk.