iris: turn the phone bind-group-layout crash into a diagnostic, drop force-gles from phone builds

UiRenderNode::new used to let a wgpu validation error reach the default
uncaptured-error handler and panic, which is what aborted the P0 bench APK
on Iris's phone in AndroidRenderer::new with only "wgpu error: Validation
Error" surviving into the truncated crash report. It now wraps creation in
wgpu error scopes and returns Result<Self, String>; the Android backend
turns a failure into the adapter's identity, the limits/downlevel flags a
layout validates against, and wgpu's own error chain, logged as one logcat
line and shown on screen (IrisView.showRendererError) instead of crashing.

Auditing every bind-group-layout entry against wgpu-core's own validation
source names the likely cause: masks_layout's move_offsets storage buffer
is visible to the vertex stage, which Vulkan grants unconditionally but
GLES gates on the driver's own vertex-stage SSBO support -- and the
delivered APK was built with force-gles, a flag meant only to force the
*emulator* onto GLES for one frame-time measurement, that build-apk.sh's
default feature list applied to every arm64 build regardless of target.
Its default no longer includes force-gles.

Testing the diagnostic (by inducing an artificial validation error) also
found and fixed a real reentrancy bug: calling Activity.setContentView
synchronously from inside a ViewPeer callback re-enters the same peer's
RefCell borrow through onFocusChanged, aborting with "RefCell already
borrowed". Deferred through the same push_dynamic_deferred_callback
mechanism raise_if_enabled already uses.

Full audit, verification, and the named hypothesis are in RUST.md's P0
box ("iris bench crash on the phone, 2026-09-06"); the API change is in
IRIS.md. Nobody on this session has the phone, so this is unconfirmed
against real hardware -- the point of (1) is that the next run says so
either way.

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

docs/IRIS.md

@@ -8,6 +8,35 @@ capability that moved. Small and trivial changes do not go here.
 An entry gives the date, what changed, why, and a short before/after where
 it helps judge the change without the session that made it. Newest first.
 
+## 2026-09-06: `UiRenderNode::new` returns `Result`, not `Self` (RUST.md's P0 box, phone-crash fix)
+
+`iris_core::UiRenderNode::new(device, queue, config)` now returns
+`Result<Self, String>` instead of `Self`. Why: it used to let a bind-group-
+layout validation failure reach wgpu's default error handler, which panics
+with no way for a caller to intervene -- exactly what aborted the P0 bench
+APK on Iris's phone with the crash report truncated to "wgpu error:
+Validation Error" and nothing else recoverable. It now runs its creation
+calls inside wgpu error scopes and returns the full error text (wgpu's own
+"Caused by" chain) as `Err` instead.
+
+Both callers changed to match: `android::render::AndroidRenderer::new`
+itself now returns `Result<Self, String>` too, building a fuller report
+(adapter identity, the limits/downlevel flags a layout validates against,
+then wgpu's text) on failure -- its caller,
+`android::view::IrisViewPeer::surface_changed`, logs that report as one
+logcat line and shows it on screen (a new `IrisView.showRendererError`,
+called via an ordinary JNI method call rather than a new `native fn`)
+instead of letting the process abort. `default::render::UiRenderer::new`
+(the winit/desktop backend) still panics on failure -- there is no
+on-screen fallback there -- but the panic message is now the same full
+text rather than whatever wgpu's own handler would have printed.
+
+No change for an app that never constructs a `UiRenderNode` directly (every
+current one goes through `AndroidRenderer`/`UiRenderer`), but anyone who
+does needs an `?`/`.expect()`/`match` at the call site now. Full audit and
+the named hypothesis for what actually failed on the phone are in
+RUST.md's P0 box, "iris bench crash on the phone, 2026-09-06."
+
 ## 2026-09-05: `AndroidAppState::platform_ready` (RUST.md's P0 box, iris half)
 
 Added a second, optional lifecycle method to `iris::android::AndroidAppState`