Fling: the vsync clock, the frame ask, and a report that can say what it measured

Iris, from her phone: "some stuttering when flinging in particular.
Harder to notice with my finger directly moving the scroll." Her fling
phase was 103fps on a 120Hz screen at p50 6.3ms.

Two of the four things found are corrections to the instrument, not the
renderer. The swapchain acquire -- `get_current_texture`, which *blocks*
until the compositor frees an image -- was inside the span the report
called iris's CPU work, so a fling comfortably ahead of the display read
as milliseconds of being slow. A frame is now three measured parts
(`FrameParts`: build, acquire, submit), per phase as well as per run. And
nothing could say a frame was never *produced*: `late` counts frames that
cost too much, which a reader does not see, while a frame that never
happens leaves the last one up for two refreshes, which is the stutter.
`PhaseStats::missed` counts vsyncs nothing was drawn for. It closes on
the emulator: 1548 frames + 452 missed over 33.0s at 60Hz is 1980
vsyncs.

The other two are the frame loop. `Choreographer.postFrameCallback`
schedules for the next vsync after the call, and iris asked at the *end*
of the callback -- so any frame whose work ran past the boundary
registered too late and got the vsync after, one frame over budget
silently costing a second. It is asked for immediately after
`tick_animations` now, on both backends. And the fling was advanced on
`Instant::now()` rather than the vsync `do_frame` carries: frames are
presented on an even cadence whatever clock computes them, so sampling
the spline at "whenever the callback ran" moves the content unevenly with
no frame late enough to appear in any report -- and a drag never had it,
which is the asymmetry Iris described. `PointerClock` is `DeviceClock`
and the view keeps one, anchored by whichever of a touch or a frame comes
first, so a fling is advanced on the clock its velocity was measured on.

`opt-level` for the Android release build goes from "s" to 3. The table
in RUST.md picked "s" on bytes alone; over the same warm fling eight
times iris's own per-frame work is p90 0.15ms/p99 0.42ms at "s" against
p90 0.09ms/p99 0.26ms at 3, for 1.8 MB of arm64 APK.

`app-rust/tests/fling_profile.rs` is the rig that established what a
fling frame actually costs and is kept for next time (Iris: "please keep
the profiling rig around for future use"): only one frame in six lays
anything out, and the multi-millisecond spikes are all first-pass.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/RUST.md

@@ -423,6 +423,71 @@ vectorisation on a renderer. Everything else is
 own rather than `release`, so the desktop build is not also optimised for
 size.
 
+### The fling stutter, and what a frame report could not say (2026-09-09)
+
+Iris, from her phone: *"I'm noticing some stuttering when flinging in
+particular. Harder to notice with my finger directly moving the scroll."*
+Her report had the fling phase at 3396 frames over 33.0s -- 103fps on a
+120Hz screen -- with p50 6.3ms and 13.4% "late".
+
+**The report was not measuring what its own labels claimed.** Three
+things came out of chasing it, and the first two are corrections to the
+instrument rather than to the renderer:
+
+1. **The swapchain acquire was counted as iris's CPU work.**
+   `AndroidRenderer::draw` timed `queue.submit` + `present()` and called
+   everything before it `redraw_to_submit`, but `get_current_texture` --
+   which *blocks* until the compositor frees an image -- sits in that
+   span. An app comfortably ahead of the display spends most of every
+   frame there, so a healthy fling read as several milliseconds of iris
+   being slow. A frame is now three measured parts (`FrameParts`:
+   `build`, `acquire`, `submit`), per phase as well as per run, because
+   they do not divide the same way in every phase.
+
+2. **Nothing could say a frame was never produced.** `late` counts frames
+   that cost more than a budget, which is not the thing a reader sees:
+   a frame that is late but drawn shows up on the next vsync, while a
+   frame that never happens leaves the previous one on screen for two
+   refreshes. `PhaseStats::missed` counts vsyncs nothing was drawn for,
+   from the gap between consecutive frame times.
+
+3. **The frame loop asked for its next frame after doing the work.**
+   `Choreographer.postFrameCallback` schedules for the next vsync *after
+   the call*, so any frame whose work ran past the vsync boundary
+   registered too late for the next one and got the one after -- one
+   frame over budget silently cost a second frame as well. It is asked
+   for immediately after `tick_animations`, before the layout and the
+   draw.
+
+And one that is about the animation rather than the report: **the fling
+was advanced on `Instant::now()`, not on the vsync the callback carried.**
+`do_frame`'s `frame_time_nanos` was discarded. Frames are *presented* on
+an even cadence whatever clock they are computed on, so sampling the
+spline at "whenever the callback got to run" moves the content by an
+uneven distance every frame -- a shimmer with no frame late enough to
+appear in any report, and it is exactly the asymmetry Iris described,
+since a drag's positions come from the finger's own timestamped samples
+and never had it. `sense::PointerClock` is now `sense::DeviceClock` and
+the view keeps **one**, anchored by whichever of a touch or a frame
+arrives first, so a fling is advanced on the clock its velocity was
+measured on.
+
+What the CPU side is *not*: `app-rust/tests/fling_profile.rs` (AGENTS.md's
+rig list) puts iris's own per-frame work during a warm fling at p99
+0.26ms, with only one frame in six laying anything out at all. The
+multi-millisecond spikes are first-pass only.
+
+### The Android release profile is `opt-level = 3`, not `"s"` (2026-09-09)
+
+The table above was measured in bytes only. `"s"` costs the loop
+vectorisation and inlining a renderer runs on: over the same warm fling
+eight times, iris's own per-frame work is p90 0.15ms / p99 0.42ms at
+`"s"` against p90 0.09ms / p99 0.26ms at `3`. The arm64 release APK goes
+from 9,745,704 to 11,542,646 bytes (+1.8 MB) -- the same trade the table
+refused for `"z"`, one level further up. Iris raised it herself
+(*"I'd make sure it's in release mode"*); the build always was, and this
+was the part of "release" that was not about speed.
+
 ### Platform fonts, not bundled ones (2026-09-07)
 
 Iris: *"remove the font for now; just match what compose does."* The