iris: the GPU test's crash was the Vulkan loader unloading Mesa, not wgpu

`mask_sdf` SIGSEGVd after printing `test result: ok`, and the workaround
was to hand the device to the process with `mem::forget` on the reading
that "dropping a wgpu device on Venus segfaults". Every part of that
except the symptom was wrong.

`rigs/gpu-probe`'s new `teardown` bin is the experiment, one variable per
mode: the same open-and-close exits 0 on the main thread and SIGSEGVs on
a spawned one; it needs no GPU work and no device, only an instance; raw
`ash` does it with no wgpu involved at all; and keeping the instance
alive fixes it. Destroying the last VkInstance makes the loader dlclose
the ICD, and Mesa's ICD here registers a pthread_key_create destructor
into its own text without `-z nodelete`, so glibc calls it through
unmapped memory when the thread exits. libtest runs every #[test] on a
spawned thread, which is the whole reason this looked like a drop bug.
`VK_LOADER_DISABLE_DYNAMIC_LIBRARY_UNLOADING=1` confirms the mechanism.

So the fix is one `wgpu::Instance` for the process -- what wgpu asks for
anyway -- and the device, queue and everything else drop normally again.
The escape and its paragraph of reasons are gone.

Also: the machine-level graphics notes duplicated in docs/RUST.md,
run-headless.sh and two source comments now point at the
`this-machine-graphics` skill, which is the only copy.

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

iris/run-headless.sh

@@ -33,10 +33,10 @@
 # through positionally without disturbing the existing `-- cargo args`
 # convention above.
 #
-# The VM has a virtio-gpu render node (Vulkan 1.4 through Venus, GL 4.6
-# through virgl), so wgpu runs on the host's real GPU -- what is missing is
-# only a compositor to give winit a surface. So: a headless sway, the same
-# trick `emu` uses for the Android emulator, and `grim` to see the result.
+# The VM has a real GPU and no display (the `this-machine-graphics` skill
+# says what it is and how it fails), so what is missing here is only a
+# compositor to give winit a surface. So: a headless sway, the same trick
+# `emu` uses for the Android emulator, and `grim` to see the result.
 #
 # It is deliberately *not* `emu`'s compositor. sway tiles, so adding a window
 # to the one an emulator is sitting in resizes that emulator's window, and a