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>

AGENTS.md

@@ -283,6 +283,20 @@ Each exists because something was invisible without it.
   checkout's own emulator, taps "Run benchmark" by label, and prints the
   report -- written so the P0 build/install/tap/read-report cycle stops
   being retyped by hand each time (docs/RUST.md's P0 box).
+- **iris's three test layers** (docs/RUST.md's "Three test layers" has
+  the commands and what each cannot answer): test at the cheapest one
+  that can answer the question. `cargo test -p transcript-fixture` runs
+  the real transcript screen over the bench fixture with **no window, no
+  compositor and no GPU** (`iris::harness`), on a clock the test owns and
+  a gesture replayed from a `t_ms action x y` file under
+  `iris/transcript-fixture/touch/` -- which is how the batched 120Hz
+  flick a finger actually makes is testable at all, since a `ui-trace`
+  swipe is many evenly-spaced events. `iris/run-headless.sh phone --phone
+  --shot …` opens the same screen in a window at the phone's own size and
+  density for looking at, and `--replay FILE` drives the same recording
+  into it. The emulator is for JNI, the IME, insets, the surface
+  lifecycle and one verification run before a build goes to the phone --
+  not for iterating on layout.
 
 ### Driving the UI