Move the settled layout vocabulary into LAYOUT.md and prune the handoff

The handoff had grown a chronicle of what each commit renamed. What is
durable about it -- the three names, and the one argument a container uses
to say them -- is design, so it goes to `docs/LAYOUT.md` as its own section
and the handoff keeps a pointer plus where the branch stands.

`docs/PLAN.md`'s "frame/extent experiment" entry described a prototype in
retired words; it becomes a pointer, and the one finding it held that was
not recorded elsewhere -- why the extent-relative child-frame trial was
removed -- joins the failed hypotheses in `docs/LAYOUT.md`.

docs/PLAN.md

@@ -864,34 +864,16 @@ such as corner radius and text's position within horizontal overflow use
 `Len`. Flexible padding participates in the same proportional allocation as a
 span rather than silently discarding its `rest` component.
 
-**The frame/extent experiment keeps measurement and geometry references separate**
-(2026-09-17, prototype only; verification and costs in `HANDOFF.md`). The parent
-chooses the reference for fractions; placing a widget's reported size does not
-replace that reference. A measurement also records the available extent, so
-reusing it checks both inputs. Retained primitive and mask coordinates name
-whether they follow the frame or the extent. Numeric extent reads constrain
-layout validity; merely drawing in an extent does not require another widget
-evaluation when it moves. Widgets also retain their original local frames,
-so reuse composes positions in the same order as a fresh draw instead of
-recovering fractions by division. The experimental oracle requires exact
-pixel-region equality. `Widget::draw` remains the only layout body. The
-prototype has significant container-update costs and is not yet adopted into
-the pinned framework. A container's length query can evaluate the offered extent
-without first placing the child's reported size; the parent's subsequent draw
-assigns the final slot using the same `Widget::draw` body. A parent-selected slot
-still honors a fixed child's alignment within that slot. Answer dependencies
-are collected only from child sizes actually read; drawing dependencies
-include all painted children. Root resize and local propagation check both
-contracts. An unchanged answer with a wider validity range retains its old
-contract: the parent's guarantee is still valid, and narrowing back must not
-trigger layout. The same rule applies to retained drawing validity. A changed
-declared size defers to its direct parent, then ordinary bottom-up propagation
-stops when both contracts remain unchanged. Blanket ancestor invalidation is
-unnecessary. The extent-relative child-frame API trial was removed: the major
-size/resize cost came from conflating these dependencies, not from Pad and
-Stack's conservative raw-placement reads. Cold layout and widespread content
-changes still perform excess provisional work; that residual has not been
-established as necessary for correctness.
+**The one-ask layout design is settled, and lives in `docs/LAYOUT.md`**
+(2026-09-17 to 2026-09-19). What this entry used to describe as the
+"frame/extent experiment" is Iris PR #19, and it has its own vocabulary now:
+the box a parent asks a widget in is its *region*, where the drawing ends up
+is its *placement*, and the length a fraction resolves against is its *rel
+base*. A container says all three in one `PlaceDesc` argument. `Widget::draw`
+remains the only layout body. The invariants, the measured costs, the fuzzing
+method and the failed hypotheses are all in `docs/LAYOUT.md`; the app's pin at
+`32f6ad8` predates every part of it, so nothing here depends on it until the
+integration described in `docs/HANDOFF.md` lands.
 
 **A `Span` leaves its children in the offered orthogonal region unless
 explicitly compacted** (2026-09-12). It still reports the widest orthogonal