Record the Remap contract and the measured resize cost

docs/IRIS_EXTRACTION_HANDOFF.md

@@ -9,7 +9,7 @@ Canonical `main` is **`43ce8c7`** (#12, pointer routing). Fourteen slices are
 in.
 
 - **#16** `split/16-draw-size`, worktree `/home/bob/repos/iris-pr16`, head
-  `984f482`. A widget sizes itself while drawing; `SizeCtx`, the `desired_*`
+  `53e6128`. A widget sizes itself while drawing; `SizeCtx`, the `desired_*`
   methods and the size cache are gone, and `on_resize(axis)` drives the
   retained path. Two review rounds answered: `Widget::draw` returns the `Size`
   (no `Painter::set_size`), both `place` methods are folded back into
@@ -33,14 +33,20 @@ readers of the axes that moved. Offered on #16 as its own slice, ahead of the
 position chain.
 
 **What the resize change taught, and why the chain moved up.** `mov` cannot
-stretch a drawing out of a box with no relative extent: `UiScalar::within` puts
-a part into such a box as a plain offset from its start, so the part's own
-fraction is gone, and `outside` asks `lerp_inv` to invert a zero-length range --
-where `div_or`'s fallback returns a rel of 0 rather than saying it cannot. The
-remap then silently leaves the drawing its old size. #16 makes `OnResize::Scale`
-refuse that case, which costs a redraw of any fixed-size subtree whose box later
-changes length. Separating the drawn box from the offered one is what removes
-the cost, so the position chain now sits directly after built-in alignment.
+stretch a drawing out of a box with no relative extent. `UiScalar::within` maps
+a part's `(rel, abs)` through a matrix whose determinant is the box's own rel
+extent, so at zero extent it is singular: every fraction of that box collapses
+onto one stored value, and nothing recovers which one. `lerp_inv` used to return
+a plausible number there instead of saying so; it and `outside` now return
+`Option`, and `Remap` is the checked region-level operation, so a drawing that
+cannot be taken out of its box is redrawn rather than left at its old size.
+
+Counted draws on a resize, against the old wipe-everything: 101 widgets of
+padded rows went 101 -> 0, `tabs` 73 -> 0, and the `text` example 49 -> 48. The
+last is the shape that does not benefit -- both paragraphs rewrap to a different
+height and the relayout runs to the root. Holding each part's own region against
+its box and resolving late is what removes those, so the position chain now sits
+directly after built-in alignment.
 
 Check for a review before starting anything, and read the newest
 `submitted_at` rather than the first result: