Measure step 5's span pin and say what blocks the sound version

Re-measured the work counters at a888717 (unchanged from adbedaf), and
tested the handoff's claim about a positive span pinning `far`: without the
pin, seed 1's resize goes from 44 draws of 13 widgets to 20 of 9. The pin is
worth removing, and what stops a sound removal is that no `Part` can say
"from the cursor to the end of the box" without naming the box's length.

docs/HANDOFF.md

@@ -254,7 +254,7 @@ Unchanged; see `docs/LAYOUT.md`.
 ## Implementation plan
 
 Work in `/home/bob/repos/iris-layout-experiment` on `wip/one-ask` from
-`a30971e`. Make each step a warning-clean commit and run its named checks
+`a888717`. Make each step a warning-clean commit and run its named checks
 before the next. If a step exposes a different mechanism, stop and update
 this handoff rather than papering over it.
 
@@ -302,9 +302,11 @@ is worth a name.
 ### 5. Restore the expected retained cost
 
 Work counters, seed 1 and 13, depth 8, widget draws / distinct widgets,
-beside `e44dea3` (#18) and `0ef87eb` (before the frame became a length):
+beside `e44dea3` (#18) and `0ef87eb` (before the frame became a length). The
+`a888717` column was re-measured after the step 1 review and is identical to
+`adbedaf` at every phase, so the scroll fix cost nothing:
 
-| seed 1  | e44dea3 | 0ef87eb | adbedaf |
+| seed 1  | e44dea3 | 0ef87eb | a888717 |
 | ---     | ---     | ---     | ---     |
 | cold    | 369/261 | 331/288 | 342/288 |
 | many    | 157/95  | 110/92  | 118/95  |
@@ -312,12 +314,21 @@ beside `e44dea3` (#18) and `0ef87eb` (before the frame became a length):
 | scroll  | 2       | 1       | 1       |
 | resize  | 13/13   | 40/15   | 44/13   |
 
-| seed 13 | e44dea3  | 0ef87eb  | adbedaf  |
+| seed 13 | e44dea3  | 0ef87eb  | a888717  |
 | ---     | ---      | ---      | ---      |
 | cold    | 1330/707 | 1179/982 | 1278/982 |
 | many    | 524/159  | 424/364  | 429/366  |
 | resize  | nothing  | nothing  | nothing  |
 
+The command is
+
+```sh
+IRIS_SEED=1 IRIS_DEPTH=8 cargo test --release --features layout-diagnostics \
+  --test layout_diagnostics -- --ignored --nocapture layout_cost
+```
+
+reading `widget draws` and `distinct widgets` from each phase's block.
+
 `many`, `size` and `scroll` are better than #18 and within a few draws of
 `0ef87eb`; `cold` is 3% and 8% more than `0ef87eb` for the same distinct
 widgets, which is the frame pins making a widget answer again where it used
@@ -348,7 +359,37 @@ Three mechanisms behind what is left, all understood:
 - **A positive-direction span with no shares pins `far`** it does not need,
   so a resize redraws it. Read `extent_len` only where a slot depends on it
   (shares, or `Sign::Neg`), and express the measuring room's far end without
-  the length. Measure `resize` at seed 1 before and after.
+  the length.
+
+  **Measured at `a888717` (worker, 2026-09-19)**, by replacing `far` with a
+  deliberately unsound read that returns the length without pinning: seed 1's
+  `resize` goes from **44 draws of 13 distinct widgets to 20 of 9**, and the
+  median frame from 0.063 ms to 0.022 ms. So the pin is worth the step, and
+  it is the pin rather than the twice-asked multiplication -- the hottest
+  widgets are drawn 8, 8, 4, 4, 4, 4, 2, 2 times, and only 3 of 98 reuse
+  attempts are rejected by a frame length. A symbolic pin fails on a resize
+  because a symbolic length is not window-independent: a scroll's content box
+  is `rel 0, px content_len`, and `content_len` is measured against the
+  window.
+
+  **What blocks the sound version.** The measuring room is
+  `Part::From(UiSpan::new(cursor, far))`, and no existing `Part` says "from
+  the cursor to the end of the box" without naming the box's length. A new
+  variant would -- an inset of `lead` and `trail` *window* lengths, which
+  also subsumes `Part::Of`'s pure-pixel use in `Pad` -- but `in_parent` needs
+  an arm for it, and the mapping is not a plain `Holds::through`: the inset
+  is a window length, so the child's box length changes with the window and
+  the offset cannot be written as a `Len` of the parent's box.
+
+  There is a second, deeper question under it. Dropping the pin is only
+  sound if nothing else has to notice the span's box changing, and today
+  `in_parent`'s `_` arm converts a child's box range into a range on the
+  *window* through the child's box length -- which assumes that length is a
+  fixed expression of the window, which is exactly what `far` changing
+  violates. Putting that dependency on the parent's own box instead would
+  let the span stop reading `far` for soundness and read it only where a
+  slot depends on it, but it will also invalidate more elsewhere. Decide
+  that before writing the `Part` variant.
 - **A rule that is a fraction of the frame pins the frame**, which is what
   `cold` grew by. The answer for such an axis is a pure function of the rule
   and the frame, so a reuse could resolve it again from the record instead