Correct the frame-as-length step: Part::of is untouched, movement paths merge

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/HANDOFF.md

@@ -283,22 +283,19 @@ pub struct Painter<'a> {
     ...
 }
 
-impl Part {
-    /// Where it lands in the coordinates `extent` is in. A `From` span is in
-    /// the asking widget's frame lengths, which `frame` says in the node's.
-    pub(crate) fn of(self, extent: UiSpan, frame: Len) -> UiSpan {
-        match self {
-            Self::All => extent,
-            Self::From(span) => UiSpan::new(
-                extent.start + span.start.within_len(frame),
-                extent.start + span.end.within_len(frame),
-            ),
-            Self::Of(span) => span.within(&extent),
-        }
-    }
-}
 ```
 
+`Part::of` does not change: with one unit a `From` span is already a
+window-unit length, so a span's cursor, `far`, `room` and slots stay as they
+are and only its shares boundary reads `room.to_px(window)`. Two places
+resolve through a frame: a reported fraction (`in_parent_frame`, now
+`size.within_len(child_frame)`) and a declared one (`narrowed_by`, then a
+`Sized` part placed by alignment). A widget's own frame reads convert once,
+`Holds::at(frame_px).through(frame)`, the exact preimage of the `to_px` that
+made them. `recompose_subtree` and `reposition` become one operation, since
+a child's box is always `Part::of(parent box, place)`; `asked_px` is the
+window.
+
 A child's frame is `narrow.within_len(parent.frame)` where the caller or a
 rule narrowed it, else the parent's; a declared length also decides the box
 (`len` placed in the part by alignment, as `frame_and_extent` does today),
@@ -329,8 +326,6 @@ What this changes in the core, and why it is not merely more code:
 - `in_parent` no longer maps a child's frame holds through the frame length
   per level: every widget holds for the window's pixels, so the holds `and`
   directly, at region nodes too.
-- `Part::of` gains one `within_len` per `From` end. `Of` and `All` are as
-  they were.
 - `DrawInfo::frame`/`frame_abs`, `ActiveData::frame`/`frame_abs`, `local`
   in `Placing`, `recompose_subtree` and `asked_px` all change meaning:
   `frame` is a `UiVec2` length, `frame_abs` goes, a node's move entry is a