iris: FrameReport CPU/GPU split, force-gles backend switch, iris-scroll.sh rig

Splits each frame sample at queue.submit into redraw-to-submit (iris's own
CPU work) and submit-to-after-present (driver/GPU wait), so RUST.md's I5
"where does iris's frame time go" question can be answered with a number
per half instead of a single total. Adds a force-gles Cargo feature that
switches the Android wgpu::Instance from Backends::PRIMARY to Backends::GL
at compile time (no runtime env-var path exists into an already-launched
Android process on this machine), for isolating SwiftShader-Vulkan vs.
GLES/virgl as the software-mode gap's cause. app/iris-scroll.sh extracts
transcript-bench.sh's exact 24-swipe/6-cycle gesture loop for iris's own
demo app, which transcript-bench.sh cannot drive directly since it opens a
session through the Compose app's own UI.

Verification (this pass, on a disk-pressure-limited host running low on
space): cargo fmt --all clean, no diff. cargo clippy --workspace
--all-targets: no warnings from this diff (pre-existing future-incompat
notices from wgpu/winit/naga only). cargo test --workspace and cargo ndk
for iris-android-app --features transcript-screen were verified clean by
the previous pass on this identical diff (fmt/clippy/test/ndk all clean,
per that pass's own report); not re-run here because the host's disk was
93% full and a concurrent ai-server rebuild (stable toolchain moved to
1.98.1, rebuilding aws-lc-sys from scratch) had driven I/O pressure to
~60%, so a repeat cargo test --workspace sat 50+ minutes doing no useful
work and was stopped rather than left to make the disk situation worse.

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

iris/android-app/Cargo.toml

@@ -37,6 +37,10 @@ serde_json = { version = "1", features = ["float_roundtrip"], optional = true }
 default = ["tabs-screen"]
 tabs-screen = ["dep:tabs-ui"]
 transcript-screen = ["dep:transcript-ui", "dep:client-core", "dep:event-model", "dep:serde_json"]
+# RUST.md's I5 "Where iris's frame time goes": forces the GLES backend
+# instead of SwiftShader's software Vulkan. See `iris/Cargo.toml`'s own doc
+# on the feature this forwards to.
+force-gles = ["iris/force-gles"]
 
 [profile.release]
 panic = "abort"