Record decided-box oracle blocker

docs/LAYOUT_LOG.md

@@ -65,6 +65,75 @@ evaluations of a ten-widget subtree in one cold layout, from `Within(cursor..
 far)` measuring rooms followed by `Fill(slot)` placing, with the span's
 `extent_len` pin forcing a redraw at every box length.
 
+## Original step 3 plan, preserved verbatim (planner, 2026-09-18)
+
+This is the original idea from ai-app-2 `280fad7`, kept here even as the
+handoff's stop notice changes with new evidence:
+
+> ### 3. Evaluate children in parent-decided boxes
+>
+> Change placement so an answer-derived box never runs a container body.
+> Placing becomes reuse-or-translate. Remove the offer/measurement gate and
+> its retained bookkeeping only as each caller stops needing it; do not leave
+> a parallel old path.
+>
+> Three current widgets must stop depending on measuring boxes they will never
+> own:
+>
+> - `Span`: do not read `extent_len` unconditionally. Slots depend on `far`
+>   only when shares exist (the decided slot fills its part) or for negative
+>   direction; compute negative-direction slots from `total`. Pin the extent
+>   length only in those cases.
+> - `Stack`: draw non-sizing children in `From(0..size)` on an axis where the
+>   sizing child's answer is `px`/`rel`, and `All` where it is `leftover`,
+>   instead of drawing them in `All` of the measuring room.
+> - `Branch` in the random rig: express "the rest of my box" as
+>   `Of(40px..FULL)` rather than reading `extent_len(Y)`.
+>
+> Every drawing must hold for the answer box it supplies. The two focused tests
+> from step 1 and the existing region-node and decided-box tests must pass here.
+
+The surrounding seven-step implementation plan remains in `docs/HANDOFF.md`;
+the quoted text above is the part whose interpretation is now blocked, so it
+must remain available even if the live handoff is rewritten.
+
+## Step 3 experiment after verifying the tests (worker, 2026-09-18)
+
+The initial 17 suite failures mixed obsolete expectations with real defects.
+`ReadsBox`, `ReadsWidth`, and `Measured` used pixel reads to compute only their
+answer; failing `Counted` cases installed a box dependency without drawing
+anything. Those fixtures encoded the old second draw inside the answer box.
+Giving the painter answer-only pixel reads preserved their invalidation and
+geometry claims while removing the obsolete paired-draw count. Separate
+fixtures still exercise actual drawing dependencies with ordinary pixel
+reads.
+
+The experiment then reached a green 114-test suite. Its substantive findings:
+
+- answer validity and final-drawing validity must remain independent all the
+  way through `draw_inner`; returning their old combined contract falsely
+  pinned wrappers such as `Pad` to provisional boxes;
+- a repeated ask of one child replaces that child's provisional final-drawing
+  contract rather than intersecting both drawings forever;
+- provisional `Span` asks fill their measuring room; `Within` immediately
+  tries to put the child in its own answer before the span decides anything;
+- only an actually allocated share pins final span slots to `far`; an
+  overfull row with a requested but unallocated share uses fixed slots;
+- a narrowed share frame is `Part::From(slot)` of the span's *extent*, not a
+  zero-based span in the forwarded frame;
+- a local redraw re-asks the question that produced the retained answer and
+  marks the parent to restore any distinct final placement.
+
+The release fast oracle then failed all eleven ordinary cases during cold
+layout. Most failures were `Text`: its provisional drawing was pinned to a
+pixel width that did not include the smaller box its answer selected. A
+generated `Stack` showed the same class through a symbolic `FULL` pin. This
+is stronger evidence than the original suite failures: the generated tests
+are not asserting draw counts or the old two-draw protocol, and no warm state
+exists yet. The open decision is whether the guarantee quoted above applies
+to every leaf and how `Text` can establish it, or whether answer-derived leaf
+evaluation remains legal while container evaluation does not.
+
 ## What the plan of 2026-09-18 got wrong (planner, same day)
 
 The plan claimed the retained model rests on the frame's length being the