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/Cargo.toml

@@ -57,6 +57,16 @@ send_wrapper = "0.6.0"
 # installs it -- this crate never installs a logger itself.
 log = "0.4.28"
 
+[features]
+# RUST.md's I5 "Where iris's frame time goes" diagnosis: forces the Android
+# `wgpu::Instance` to `Backends::GL` instead of `Backends::PRIMARY`, so the
+# same build can be measured against SwiftShader's software Vulkan ICD (the
+# default) or virgl's GLES path, without a second env-var plumbing path that
+# nothing on this machine can hand to an already-launched Android process
+# (there is no `am start` environment and no system-property reader here to
+# add one). Android-only; `android/render.rs` is the only reader.
+force-gles = []
+
 [dev-dependencies]
 tokio = { workspace = true, features = ["sync", "rt", "rt-multi-thread", "time"] }
 # The tabs example's widget tree. A dev-dependency cycle back to this