iris: fix the intermittent touch-scroll dropout (missed ACTION_DOWN hit-test)

Root-caused via temporary logcat tracing (touch events, DragArbiter state,
Selection::drag dispatch), reproduced against a real sandbox session: a
gesture's ACTION_DOWN can land on a row's own padding/gap or its header,
which CursorSense has no sensor over, so the widget that ends up handling
the gesture only ever sees Pressing frames and DragArbiter never gets
press_start -- leaving it stuck in Idle (answers Undecided forever) for the
rest of that gesture. Not the previously-suspected coalesced first
ACTION_MOVE, which is now ruled out.

DragArbiter::is_idle() lets Selection::drag notice a Pressing frame with
no matching press_start and recover the press there instead. Four new unit
tests, one of which fails on the pre-fix code.

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

docs/IRIS_TODO.md

@@ -251,6 +251,20 @@ order and what "done" looks like. Tick and date them in place.
     --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".
+  - [x] **Intermittent touch-scroll dropout — root-caused and fixed,
+    2026-09-05.** Not the coalesced-`ACTION_MOVE` hypothesis the earlier
+    pass suspected (ruled out): a gesture's `ACTION_DOWN` can land on a
+    row's own padding/gap or its header, which no `CursorSense` covers,
+    so `DragArbiter` never gets `press_start` and sits in `Idle`
+    (answers `Undecided` forever) for that whole gesture. Fixed via a new
+    `DragArbiter::is_idle()` that `Selection::drag`
+    (`transcript-ui/src/selection.rs`) checks to recover a missed press
+    on the next `Pressing` frame. Four new unit tests. See RUST.md's I5
+    box, "Touch-scroll dropout root-caused, 2026-09-05", for the trace and
+    what a peer session sharing this checkout's emulator mid-pass
+    prevented from being re-verified end-to-end (the aggregate
+    `iris-scroll.sh` three-run confirmation and a re-taken FrameReport
+    row) — a future pass should finish that once the emulator is free.
   - [ ] **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 —