RUST.md, DECISIONS.md: P0, the phone benchmark gate Iris asked for before P1

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

docs/DECISIONS.md

@@ -7,6 +7,21 @@ marked **DEFERRED** are ones the agent chose not to decide alone.
 
 ## 2026-09-05
 
+- **P0, a phone benchmark gate before any porting, asked for by Iris
+  2026-09-05**: "before P1 I'd like to see benchmarks & also maybe stress
+  test on my own phone ... If it doesn't match compose reasonably well then
+  I don't think I'd wanna continue." Design (RUST.md's P0 box has the
+  detail): the same embedded synthetic fixture in both apps with no server
+  needed; the same scripted scroll loop then a streaming phase, run
+  programmatically since the phone has no usable system tracing and no
+  agent can drive it; the same report from both (frames, janky %, p50/p90/
+  p99, process CPU time, peak RSS, battery current where readable) with a
+  copy button; the iris app under its own id and the Compose one as a new
+  `bench` build type with an id suffix, so neither replaces her production
+  install; two arm64 APKs plus instructions delivered under `~/host/bench/`.
+  The gate is hers: iris within a reasonable margin of Compose release on
+  p50, p99 and CPU time, no crashes, no visible stutter. If it fails, the
+  port stops.
 - **The rest of the port is one UI crate, `iris/app-ui`, grown out of
   `iris/transcript-ui` rather than started beside it.** It holds a
   `Screen` enum plus a back stack — the Rust equivalent of `AppRoot.kt`'s