docs: the verification pass over Tasks A and B, and the composer background withdrawn

RUST.md gains the pass's findings with their commits and the numbers:
the block model held under a per-character prefix property, the
size-independent hit-box defect and its fix, the tail-rebuild selection
gap, why the three new debug_asserts are whole-set, the text-shape
counter that turns "a delta costs one block" into a measurement, and the
verification bench run.

IRIS_TODO.md's "the bar's own grey background is not drawn" is
withdrawn: decoding the screencap puts it at rgb(41,40,49), full width,
y2245..y2365 -- drawn, and dark on black, which is most likely what the
earlier reading was.

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

docs/IRIS_TODO.md

@@ -223,10 +223,16 @@ agent takes them without colliding with that pass's `bench_client.rs`/
   `Scroll` itself turned out to measure the right number by a misleading
   route -- it is written against `painter.px_size()` now, and the claim
   below that it "measures against the window" was wrong.
-  **Still open, and pre-existing:** the bar's own grey background is not
-  drawn on this build (the `Stack{StackSize::Child(1)}` behind the field),
-  so the message reads as white text over the transcript. Present in the
-  build *before* this change too, so it is not the scroll area's doing.
+  **The grey background was not missing** -- that note (written here on
+  2026-09-06 and repeated as still open) is withdrawn. Re-measured the
+  same day on the same AVD by decoding the screencap rather than reading
+  it: the bar is `rgb(41,40,49)`, the declared `UiColor::new(40, 40, 46)`
+  after sRGB rounding, **full width and y2245..y2365** on 1080x2424, with
+  the field at `31,2277..1048,2329` and the 63px nav strip below it. It
+  is dark by design and sits on black, which is very likely what the
+  earlier reading was: at a glance the band and the background are hard
+  to tell apart. If it should read as a bar rather than as a slightly
+  different black, the colour is the thing to change, not the tree.
 
 ## From the phone, 2026-09-06, 11:39 (build delivered 02:07, commit 543f6d9)