docs: record the keyboard-wipe fix, the dp unit and the header fix

docs/IRIS.md's 2026-09-06 entry (public API), docs/LAYOUT.md's "Density:
Len::dp" design section, IRIS_TODO.md's density-unit item ticked, and
docs/RUST.md's P0 box gets the investigation: the keyboard-wipe
hypothesis and confirmation, the blur root cause and why the dp unit
turned out to be the same fix, the header cause, and what remains
unverified (an emulator screenshot of the keyboard fix, and Iris's real
phone).

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

docs/IRIS_TODO.md

@@ -410,8 +410,19 @@ do not duplicate it there.
 
 ## Build (asked for by Iris, 2026-09-06): a density-independent length unit
 
-- [ ] **A third length kind beside relative and pixels, so display scales
-  "just work".** Iris's words: "another length type similar to absolute &
+- [x] **A third length kind beside relative and pixels, so display scales
+  "just work".** Done 2026-09-06 — `Len::dp`/`len_fns::dp`, resolved
+  against `UiRenderState`/`Painter::density()` at `apply_rest` time; text
+  additionally rasterises at the resolved (physical) size instead of
+  scaling a low-resolution bitmap afterward, which was making text blurry.
+  `Span::gap`/`Padding` moved from `f32` to `Len` so they take `dp(...)`
+  too; transcript-ui's row/composer padding and one example migrated.
+  `em` was not added — nothing in this pass needed a text-relative unit,
+  and `dp`'s own doc says why it and physical pixels are kept as separate
+  fields rather than one the caller pre-multiplies. Not yet verified on
+  Iris's own phone at two densities (this pass had no device) — see
+  docs/RUST.md's P0 box and docs/IRIS.md's 2026-09-06 entry for what to
+  check. Iris's words: "another length type similar to absolute &
   relative, so instead there would be relative, pixels, and another unit
   like em or whatever is standard. That way different display scales
   should just work." Today a length is either a fraction of the parent