Record the narrow step left in the pixel path

It was in 5f16617's message and not here, which is the wrong place for it:
the handoff is what a session picking this up cold reads.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/HANDOFF.md

@@ -1066,6 +1066,14 @@ Queued from this work, in order:
   back up a mean of 2.8 levels, eight hundred times a frame. Composing on
   the fine grid alone cost 1,880M and 755M, so the widening was paid for
   twice over by not doing the walk.
+
+  **One narrow step is left in that path.** `Painter` still composes a
+  child's region into its own on the grid before asking for it in pixels.
+  Taking it out needs the child's region as its parent stated it, which
+  `draw_inner` is not handed -- it receives that box already composed into
+  the slot's frame, because that is the form `ActiveData` and the GPU want.
+  Worth doing after the region-node offer defect below, which is what pins
+  `Holds::through` at the moment.
   Two things paid for the widening and are the reason to keep them: a
   *length* composes on its own in two multiplies a level rather than the
   four both ends cost, since where the parent sits falls out of the