docs: the test rig's layers 1 and 2, with their commands and their limits

RUST.md's "Three test layers" section rewritten in place with what was
built: the `cargo test -p transcript-fixture` command and the five
assertions with the mutation that fails each, the `run-headless.sh
--phone [--replay …]` commands and the 15s/18s they take, and a
paragraph on what still cannot be answered below layer 3 (anything about
pixels, any frame time, anything JNI). Also the two traps that cost time
-- `swaymsg seat - cursor` reaching nothing on a compositor with no
input devices, and a leftover window tiling beside the new one so a
screenshot looks like a duplicated-primitive bug.

IRIS.md gains the public surface: `iris::harness`, `TouchScript`,
`List::fling_velocity`, the fling's clock, and the desktop backend's
move to physical-pixel layout with `content_scale`/`IRIS_SCALE`.

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

docs/RUST.md

@@ -48,7 +48,7 @@ closes it.
 In order; two builders at a time. Each is ticked here by the agent that
 closes it.
 
-- [ ] Test rig, layers 1 and 2 ("Three test layers" below). Running.
+- [x] Test rig, layers 1 and 2 ("Three test layers" below), landed 2026-09-07.
 - [ ] Fling parity with Compose, and the phone's keyboard push-up, with
   insets shown in the diagnostics overlay. Running, in a worktree.
 - [ ] Rows at the transcript's top edge: culled too early in one state,
@@ -93,7 +93,7 @@ The bench client (`android-app/src/bench_client.rs`, ~1000 lines) is
 the first thing to look at moving, since a desktop bench on the same
 fixture is layer 2 of the test rig below.
 
-### Three test layers, cheapest first (decided 2026-09-07, rig not yet built)
+### Three test layers, cheapest first (decided 2026-09-07; layers 1 and 2 built the same day)
 
 Iris's suggestion, adopted and layered: test at the cheapest layer that
 can answer the question, and go up only when it cannot. The emulator
@@ -101,43 +101,91 @@ costs minutes a cycle; the desktop window seconds; the headless harness
 runs inside `cargo test`.
 
 1. **Headless, in-process, no compositor and no GPU -- the default.**
-   `iris/src/layout_tests.rs` already builds a tree over `UiRenderState`
-   with no window, `sense.rs`'s gesture tests feed fabricated
-   `CursorState`s with their own times, and `List::tick_fling` is
-   driven by hand. Extend that into one harness that opens
-   `transcript-ui`'s screen on `app/bench-fixture` (as `bench_client.rs`
-   does on Android, no server), at the phone's logical size and
-   `content_scale` from `docs/bench/iris-phone-v2-2026-09-06.md` (2.55,
-   120Hz -- read, never typed from memory), ticks frames, and feeds a
-   **replayed touch stream** from a trivial file of `(t_ms, action, x,
-   y)` lines with `cursor.time` taken from the file. That is what the
-   emulator cannot do at all: the batched 120Hz flick from the phone
-   report becomes a deterministic test asserting on scroll offset and on
-   the `iris drag release:` velocity. Anything about layout, scroll
-   position, selection, focus or fold state is answered here, with
-   assertions rather than eyes. Nothing renders; a widget's placed
-   rectangle is the evidence.
+   `iris::harness` (`iris/src/harness.rs`), plus the fixture crate it
+   opens. `Harness::new(size, density)` builds an `Rsc`, a
+   `UiRenderState` and a state whose `FocusHost`/`OpenUrl` *record* what
+   the platform was asked for; `frame(t_ms)`/`frames_until(..)` run
+   frames on a clock the test owns, and `replay(&TouchScript)` feeds a
+   recorded gesture one sample at a time exactly as
+   `IrisViewPeer::on_touch_event` replays Android's historical samples.
+   The recordings are plain `t_ms action x y` files under
+   `iris/transcript-fixture/touch/`, and `flick-120hz.touch` is the
+   phone's own shape: DOWN, four samples 4ms apart, UP, 20ms in total.
+
+       cd iris && cargo test -p transcript-fixture
+
+   runs in about a second and asserts (a) the flick releases with a real
+   velocity (`List::fling_velocity`, which only `Released(Some(v))`
+   fills), (b) the list travels and settles inside the AOSP spline's own
+   `FlingCalculator::duration`, (c) a tap moves nothing and opens no
+   link, (d) a long-press-then-drag leaves selected text and does not
+   pan, and (e) the composer clears a simulated 1000px IME inset
+   (`Composer::set_bottom_inset`). Each was confirmed to fail without
+   its subject rather than assumed: dropping `animate(id)` from
+   `Selection::drag` -- the phone's own "fling does nothing" defect --
+   and starting the fling curve at the wall clock each fail only the
+   flick test; flinging on `Tapped` fails only the tap test; a 5s
+   `LONG_PRESS` fails only the selection test; a `set_bottom_inset` that
+   ignores its argument fails only the composer test.
+
+   **What still cannot be answered below layer 3**: nothing renders
+   here, so anything about pixels -- glyph rasterisation, the atlas,
+   stale or duplicated primitives, colour, the surface lifecycle, the
+   renderer rebuild -- is invisible to layer 1 and only *looked at* in
+   layer 2. Frame *times* are not measurable at either: layer 1 does no
+   GPU work at all and layer 2 runs a debug build on this VM's virtio
+   GPU, so a number from either is not the phone's. Anything JNI (the
+   IME, real insets, the clipboard, battery) is layer 3 by construction:
+   layer 1 records that the platform was asked and layer 2 has no
+   Android platform to ask.
+
 2. **A phone-shaped desktop window under headless sway -- for looking.**
-   `iris/run-headless.sh` already runs a winit binary under a private
-   sway and screenshots it with `grim`. Add a `--phone` mode (output and
-   window at the phone's size and scale, with the scale reaching iris
-   the way Android's does so dp layout runs at that density) and
-   touch-shaped mouse input: a left-button drag pans and flings through
-   `DragArbiter`, long-press selects, no hover. One input path, not a
-   parallel one (`CODE_RULES`). Colour, spacing, text and anything a
-   person has to see is answered here. `swaymsg seat - cursor
-   move/press/release` drives it when a gesture is needed on screen.
+
+       cd iris && ./run-headless.sh phone --phone --shot /tmp/p.png -- -p transcript-fixture
+
+   About 15 seconds warm. `--phone` sets the private sway output to
+   1080x2424@120Hz and exports `IRIS_SCALE=2.55`, which reaches iris the
+   way `DisplayMetrics.density` does on Android
+   (`iris::default::content_scale`) -- the desktop backend now lays out
+   in physical pixels with a density instead of dividing into a separate
+   logical space, so both platforms run one path. `transcript-fixture`'s
+   `phone` example opens the same screen from the same bytes as layer 1
+   and the Android bench.
+
+   A gesture on screen uses the *same recordings*:
+
+       ./run-headless.sh phone --phone --replay transcript-fixture/touch/flick-120hz.touch \
+           --shot /tmp/p.png -- -p transcript-fixture
+
+   writes `/tmp/p-before.png` and `/tmp/p.png` either side of the flick;
+   looked at 2026-09-07, the list moved back about seven turns of the
+   fixture and settled.
+
+   **`swaymsg seat - cursor` cannot drive it, and that cost an hour.**
+   This compositor runs the headless backend with no input devices
+   (`WLR_LIBINPUT_NO_DEVICES=1`, `LIBSEAT_BACKEND=noop`): the cursor
+   commands all report `success` and nothing whatever reaches the
+   client, with `swaymsg -t get_seats` showing `capabilities: 0` as the
+   only sign. wlroots 0.19 dropped `WLR_HEADLESS_INPUTS`, and ydotool's
+   uinput device would be ignored by a compositor that is not reading
+   libinput. `iris/rig-input`'s `replay-touch` uses the
+   **virtual-pointer protocol** instead, which is a client protocol and
+   needs neither devices nor root, and it parses `iris::harness`'s own
+   `TouchScript`. Two traps inside it, both found by printing winit's
+   events: a button sent in the same frame as the motion that first puts
+   the pointer over the window is dropped (the client sees the enter,
+   the moves and the *release*, never the press), so the pointer is
+   positioned and left to settle 200ms first; and a leftover window from
+   an earlier manual run **tiles beside the new one**, halving the width
+   and producing a screenshot that looks exactly like a duplicated-
+   primitive rendering bug -- `swaymsg -t get_tree` and `pgrep -af
+   examples/phone` are the check.
+
 3. **The Android emulator -- platform plumbing and the final pass.**
    JNI, IME, insets, surface lifecycle, the renderer rebuild, and one
    verification run before a build goes to the phone. Not for iterating
    on layout.
 
-Pass condition for the rig: `cargo test` runs a fixture-backed headless
-transcript screen with a replayed flick and asserts a nonzero release
-velocity and a moved scroll offset; one command opens the same screen in
-a phone-shaped window and screenshots it. Record the commands here when
-it lands.
-
 ### The 22:16 phone report, worked 2026-09-06/07
 
 Iris's four items are listed in docs/IRIS_TODO.md's "From the phone,