RUST.md: iris's bindless texture array does not survive real Android GPUs

Iris asked whether the 'unknown number of images' approach even works on
mobile. It does not, measured with a new rig (rigs/gpu-probe, no APK
needed) and sourced rather than recalled: the emulator's software Vulkan
refuses iris's descriptor-indexing request outright, and on real hardware
the current Android Vulkan Profile baseline (80.1% of active devices)
does not require VK_EXT_descriptor_indexing either -- Arm's own docs say
only Valhall/5th-Gen Mali (2019+) support it.

iris already solved the identical problem for text in I1 (the glyph
atlas). The recommendation is to generalize it to images rather than
widen the binding array further; not yet implemented, since it changes
iris's render core.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

rigs/gpu-probe/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "gpu-probe"
+version = "0.1.0"
+edition = "2024"
+
+# Deliberately its own crate rather than a member of iris's workspace: the
+# vendored `iris/` tree is meant to stay reconcilable with the iris/iris
+# repository, and this is a rig belonging to ai-app.
+
+[dependencies]
+# Pinned to what iris asks for, so the answer is about iris rather than
+# about a different wgpu.
+wgpu = "28.0.0"
+pollster = "0.4.0"
+# Queried directly, because wgpu and `cmd gpu vkjson` disagreed about
+# descriptor indexing in the emulator and only the raw call says which is
+# right.
+ash = "0.38"