iris: List::fling with Android's spline physics, and fix the drag-slop scroll jitter

Adds VelocityTracker and a port of AOSP SplineOverScroller's fling curve
(FlingCalculator, cited at the definition) to iris::sense, and wires
List::fling/is_scrolling/cancel_fling/tick_fling through
Selection::drag's release path -- a pan's release now decelerates instead
of stopping dead on the finger lifting, matching IRIS_TODO.md's "swiping
has no momentum" ask. Clamped at the loaded content's start/end and
cancelled by the next touch-down.

Also fixes the scroll jitter DragArbiter's slop release caused: crossing
DRAG_SLOP applied the whole pre-threshold drag (measured from press_start)
in one step, since nothing pans while a gesture might still resolve to a
selection. Now only the excess past DRAG_SLOP is applied on that frame,
the same way Android's own touch handling consumes touch slop rather than
replaying it.

Root-caused by reading DragArbiter's state machine and covered by new
unit tests (fling distance against the closed-form spline result within
1%, cancel-on-touch, start/end clamp, the slop-crossing regression); no
emulator was used this pass, so an on-device trace/feel-check is still
open, and Benchmark v2's four-phase bench_client.rs spec was not
attempted. docs/IRIS.md, docs/IRIS_TODO.md and docs/RUST.md's P0 box
record what's done and what's left.

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

docs/RUST.md

@@ -4438,6 +4438,26 @@ device.
       apk/release/iris-bench-arm64.apk`; that repo's own README gained a
       dated entry. Still not confirmed on Iris's actual phone.
 
+      **Fling and jitter, 2026-09-06.** The two `IRIS_TODO.md` "From the
+      phone" items this box's own text names as follow-ups are fixed --
+      `List::fling`/`VelocityTracker`/`FlingCalculator` (IRIS.md's
+      2026-09-06 entry) and the `DragArbiter` slop-release jump (fixed by
+      applying only the excess past `DRAG_SLOP` on the crossing frame,
+      not the whole pre-threshold drag) -- both wired through
+      `Selection::drag`'s release path, both covered by new unit tests in
+      `iris/src/sense.rs` and `iris/src/widget/list.rs`. **Root-caused by
+      reading `DragArbiter::update` and testing it directly, not by an
+      emulator trace** -- this pass did not open an emulator, so the
+      "trace the list's offset per frame" verification this box's own
+      todo asked for is still open, as is a feel-check of the fling on
+      real touch input. **Benchmark v2's four-phase spec (fling/stream/
+      type/keyboard) in `bench_client.rs` was not attempted this pass** --
+      wiring a real IME show/hide and refresh-rate read through
+      `bench_jni.rs`, and `FrameReport`'s per-phase accounting, is real
+      scope on its own and was left rather than shipped half-verified;
+      the Compose half above is already done and is the reference shape
+      for whoever picks this up. No redelivery this pass.
+
 - [ ] **P1 — session screen parity.** History paging backward (with the
       page-boundary healing `client-core` does not have yet, below),
       `TranscriptSource`-backed cache/server stitching, jump-to-latest,