docs/RUST.md: the emulator crash loop was the missing GLES fallback, with the panic-hook note

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

docs/RUST.md

@@ -716,11 +716,66 @@ closes it.
   devlog agent shrank the label type 18 -> 13 to make room (UI_RULES:
   never shrink text to fit). Put the controls in two rows or make the
   header wrap; restore the size. Sonnet.
-- [ ] Bench app crash-loops on this checkout's emulator once
-  backgrounded (seen by the devlog agent, pre-existing; the phone's
-  resume works since ba2afba). Reproduce on the emulator's GLES path,
-  read the crash from the devlog/`adb logcat`, fix. Opus if it is in
-  the surface lifecycle.
+- [x] **Bench app crash-loops on this checkout's emulator (done
+  2026-09-07).** Not the surface lifecycle at all, and not "once
+  backgrounded" -- a build with the **default features** (no
+  `force-gles`) never got a first frame. `AndroidRenderer::new`
+  (`iris/src/android/render.rs`) asked wgpu for `Backends::PRIMARY`,
+  which does not contain `GL`, and this emulator advertises a Vulkan ICD
+  with no adapter behind it: `RequestAdapterError::NotFound {
+  active_backends: VULKAN, no_adapter_backends: VULKAN,
+  supported_backends: VULKAN | GL }`, `.expect`ed, so SIGABRT, so the
+  launcher restarts it -- the loop. A `force-gles` build was never
+  affected, which is why the crash looked like it belonged to whatever
+  else was going on. **Root cause: iris refused a device whose only
+  usable adapter is a GLES one.**
+
+  **Fix, two halves.** (1) `AndroidRenderer::new` now probes for a
+  `PRIMARY` adapter with an instance that never touches the window and
+  rebuilds the instance on `Backends::GL` when there is none. The probe
+  is surface-free deliberately: **an Android window can be connected to
+  one graphics API only**, so a single instance carrying both backends
+  fails differently and worse -- Vulkan's
+  `vkCreateAndroidSurfaceKHR` claims the window in `create_surface` and
+  the GLES surface built from the same window then reports `In
+  Surface::configure / Invalid surface`, aborting one frame later in
+  `Surface::get_current_texture_view` ("Surface is not configured for
+  presentation"). That was measured here on the way to the fix, not
+  reasoned about. Vulkan still wins wherever it has an adapter, so
+  nothing changes on the phone. (2) The surface, adapter and device
+  requests all report through the `Result<Self, String>` this function
+  already returns, instead of two of the three panicking -- one rule for
+  the set, and `surface_changed` already puts that string on screen and
+  in the log ring.
+
+  **Evidence**, this checkout's emulator (API 36, x86_64, debug):
+  before, default features aborted on first launch with `Abort message:
+  'Could not get adapter!: NotFound {...}'`; after, `iris renderer: no
+  Backends(VULKAN | METAL | DX12 | BROWSER_WEBGPU) adapter on this
+  device, falling back to GLES` then `new renderer built (Gl)` and
+  frames. Then the cases the fix had no reason to touch, clean on both
+  the default build and a `force-gles` one: two background/return
+  cycles, rotate to landscape and back (`already_live=true`, the reuse
+  branch), a background/return after the rotation, and cold starts.
+  **Vulkan could not be exercised here** -- the probe's own answer is
+  that this emulator has no Vulkan adapter, which is the whole defect;
+  Vulkan remains only testable on Iris's phone.
+
+  **Also landed with it, and worth more than the fix**: a panic hook in
+  `iris/android-app/src/app_log.rs`. Checked first, rather than assumed:
+  under `panic = "abort"` a panic's message reaches the tombstone's
+  `Abort message` and **nothing else** -- not `log`, so not the ring, so
+  not Dev Updater's Runtime tab, which is the only surface Iris has on a
+  phone with no `adb`. The hook writes the message and its location at
+  `error` level, and -- because the ring is memory only and the process
+  is about to die -- also to `last-panic.txt` in the app's private
+  directory, which `set_crash_dir` (called from `nativeSetFilesDir`)
+  replays into the ring at `error` level on the next start and deletes.
+  So a crash loop now explains itself in the Runtime tab of the run
+  that is still up. Verified on the emulator by building the *unfixed*
+  renderer with the hook: `iris panic at .../render.rs:140:14: Could not
+  get adapter!: NotFound {...}` in the ring on the run that died, and
+  `iris app log: the previous run died -- ...` on the next one.
 - [ ] Masks with a shape -- docs/LAYOUT.md "Masks with a shape (decided
   2026-09-07)". A mask references a primitive already drawn
   (rect SDF, texture or glyph alpha), chained and multiplied; `.masked()`