iris: generalize drag arbitration into a default-input DragGesture, with pointer capture and Drop

Iris asked (2026-09-06) that dragging be part of iris's default input
system rather than duplicated per app: "anything that provides good
performance and can be generalized well is part of iris rather than the
app." DragArbiter and VelocityTracker (both already in iris::sense) are
now bundled into a new DragGesture, which also takes exclusive pointer
capture (UiRenderState::capture_pointer/release_pointer/captured_pointer)
the moment a gesture commits to panning or selecting, and delivers a new
CursorSense::Drop -- not PressEnd -- to the captured widget when the
button lifts, wherever on screen that happens to be.

This directly targets the phone bench's "finger flings do nothing":
per-widget hit testing silently drops a gesture the instant the pointer
moves off every registered region, which a fast pan/fling does routinely
(crossing several virtualised rows, or ending off the loaded content
entirely) -- so PressEnd, and the velocity/fling-start decision hanging
off it, was frequently never delivered at all. Capture targets List's own
stable id (List::key_at resolves the row-under-pointer from its
extents), not a row's, since List retires rows mid-drag as content
scrolls.

transcript-ui::Selection::drag now only decides pan-vs-select from
DragGesture's outcome; row.rs's per-row registration is only ever a
gesture's first frame, with lib.rs registering the List-level
continuation once. New tests: sense_tests.rs's two pointer-capture
regressions, list.rs's replacing_the_last_row_many_times_does_not_leak_primitives
(a P0 stale-primitives diagnostic -- passes, pinning the widget-arena
layer as not the leak). MainActivity.java opts into edge-to-edge
(Window::setDecorFitsSystemWindows(false), API 30+, no new dependency)
so window insets are redelivered on every change including a pure IME
toggle -- the named-but-untried fix for the phone bench's "keyboard:
could not be shown" and the emulator's identical non-confirmation.

cargo fmt/clippy/test clean across the iris workspace.

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

iris/android-app/app/src/main/java/dev/iris/android/demo/MainActivity.java

@@ -31,6 +31,25 @@ public final class MainActivity extends Activity {
         setContentView(layout);
         view.requestFocus();
 
+        // RUST.md's P0 box, defect 4 ("keyboard: could not be shown"):
+        // `logcat` showed the platform's own IME open/resize happening
+        // while `setOnApplyWindowInsetsListener` fired only once, at
+        // attach, and never again for a pure keyboard toggle -- a plain
+        // (non-edge-to-edge) window is only guaranteed that one initial
+        // dispatch; `adjustResize` handling the IME entirely by resizing
+        // the window is not itself a trigger for a fresh one. Opting into
+        // edge-to-edge (a platform call, API 30+, no new dependency) is
+        // what makes the system redeliver insets on every change,
+        // including the ones this activity actually cares about --
+        // `getSystemWindowInset*` below is unaffected by this (it has
+        // always reported the raw system-bar/IME overlap regardless of
+        // who consumes it), so the on-screen bars and the padding Rust
+        // already derives from those four numbers are unchanged; only the
+        // callback's firing became reliable.
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+            getWindow().setDecorFitsSystemWindows(false);
+        }
+
         view.setOnApplyWindowInsetsListener((v, insets) -> {
             int left = insets.getSystemWindowInsetLeft();
             int top = insets.getSystemWindowInsetTop();