iris: DragArbiter closes I5's touch-drag pan-vs-select gap

A row's own click_or_drag() selection handler always won the same
gesture a list-level pan wanted, since run_sensors gives the inner
layer first refusal every frame it's pressed. DragArbiter
(iris/src/sense.rs) decides pan vs. select the way Android does:
vertical drag pans immediately, a held stationary press starts a
selection after LONG_PRESS, and a horizontal drag on already-selected
text extends immediately. transcript-ui's Selection::drag routes
every row's drag through one arbiter per list, driving List::scroll
for a pan instead of a second scroll mechanism.

8 new unit tests (iris::sense::drag_arbiter_tests); cargo
fmt/clippy/test --workspace and cargo ndk (iris, transcript-ui) all
clean; run-headless.sh screenshot byte-identical to before the change.

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

IRIS_TODO.md

@@ -184,17 +184,25 @@ order and what "done" looks like. Tick and date them in place.
     `app/ui-sandbox.sh --delay` (this crate deliberately fetches nothing
     itself, `transcript-ui/src/lib.rs`'s doc), a new cdylib + Gradle
     module, then the bench script pointed at it.
-  - [ ] **Touch-drag panning over a row's own rendered text.** `row.rs`
-    registers `CursorSense::click_or_drag()` on each row's `TextEdit` for
-    cross-row selection; `TextEdit::draw`'s `painter.child_layer()`
-    (`iris/src/widget/text/edit.rs:87`) means that registration wins
-    `core/src/sense.rs::run_sensors`'s per-layer arbitration on every
-    frame it is pressed, not just the frame the press started, so a list
-    pan gesture registered on `List` itself never gets a turn while a
-    row is under the finger. Fix: a small press distance/time arbiter
-    deciding pan vs. select before either commits, or gate text-drag-
-    selection behind a long-press so an ordinary swipe always pans first.
-    `lib.rs`'s module doc has the full diagnosis.
+  - [x] **Touch-drag panning over a row's own rendered text — done,
+    2026-09-05.** `row.rs` used to register `CursorSense::click_or_drag()`
+    on each row's `TextEdit` for cross-row selection; `TextEdit::draw`'s
+    `painter.child_layer()` (`iris/src/widget/text/edit.rs:87`) meant that
+    registration won `core/src/sense.rs::run_sensors`'s per-layer
+    arbitration on every frame it was pressed, not just the frame the
+    press started, so a list pan gesture registered on `List` itself never
+    got a turn while a row was under the finger. Fixed with
+    `iris::sense::DragArbiter` (recorded in `IRIS.md`), one small state
+    machine per list deciding pan vs. select the way Android does (a
+    vertical drag pans immediately; a stationary press held `LONG_PRESS`
+    (500ms) starts a selection which further drag extends; a horizontal
+    drag while something is already selected extends immediately) —
+    `transcript-ui/src/selection.rs`'s `Selection::drag` is the one place
+    every row's drag now routes through. 8 new unit tests
+    (`iris/src/sense.rs`'s `drag_arbiter_tests`); `cargo fmt/clippy/test
+    --workspace` and `cargo ndk` (both `iris` and `transcript-ui`) all
+    clean; `run-headless.sh` screenshot byte-identical to before the
+    change (38578 bytes). See RUST.md's I5 box, "Gap closed, 2026-09-05".
   - [ ] **Row-level accessibility names.** The composer carries
     `.label("Message")`; transcript rows do not carry a `.label()` of
     their own yet, so `Widgets::named()` (I4) does not include them —