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.md

@@ -146,6 +146,24 @@ on a row's own rendered text now pans the list correctly instead of
 always starting a selection. 8 new unit tests in `iris/src/sense.rs`'s
 `drag_arbiter_tests` module.
 
+### 2026-09-05, later: `DragArbiter::is_idle()`, recovering a missed `press_start`
+
+Follow-up to the above, from a real touch-scroll dropout: a gesture's
+`ACTION_DOWN` can land on a caller's own dead space (a row's padding, a
+gap, a header with no handler) that never calls `press_start`, so the
+first frame the arbiter actually sees is a `Pressing`-shaped `update`
+with no matching start. Before this, `update`'s `Idle` arm had no way to
+tell that apart from "nothing is happening" and answered `Undecided`
+forever for the rest of that gesture. `is_idle(&self) -> bool` lets a
+caller notice the gap and recover: if `is_idle()` is true on a frame the
+caller knows a press is genuinely down (its own `Pressing`/equivalent
+sense fired), call `press_start` right there instead of assuming one
+already happened. `transcript-ui`'s `Selection::drag` is the reference
+caller — one new match arm, checked before the ordinary `update`-only
+case. Any other `DragArbiter` caller with the same "one sensor per
+sub-region, no fallback for dead space" shape has the same gap and wants
+the same recovery.
+
 ## 2026-09-05: `SpanStyle`, per-range text styling (RUST.md's I5)
 
 A `TextBuffer` used to have exactly one style (`TextAttrs`: colour, size,