iris: stop requesting compute-shader limits nothing uses

adapter.request_device asked for Limits::default(), which requests
desktop-tier compute-shader limits unconditionally even though nothing in
iris/iris-core creates a ComputePipeline or writes a @compute stage. That
crashed device creation outright on a downlevel GL adapter reporting
OpenGL ES 3.0 (no compute at all) -- the Android emulator's
EMU_GPU=software/force-gles path, and any real GLES-3.0-only device.

New iris_core::device_limits(), shared by both platform backends, zeros
exactly the six max_compute_* fields rather than switching to a downlevel
Limits preset -- downlevel_webgl2_defaults() also zeros
max_storage_buffers_per_shader_stage, which shader.wgsl's vertex stage
needs. rigs/gpu-probe's own mirrored limits were updated to match.

Not verified against the actual SwiftShader-ES-3.0 crash on-device this
pass: the cold boot needed would have force-restarted this checkout's
emulator while another session had its own app running on it.

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

docs/IRIS.md

@@ -8,6 +8,31 @@ 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.
 
+## 2026-09-05 (later still): `iris_core::device_limits()`, and iris no longer requests compute-shader limits
+
+New public function, `iris_core::device_limits() -> wgpu::Limits`. Why:
+`adapter.request_device`'s `required_limits` was `Limits::default()` plus
+a `max_buffer_size` override in both platform backends, and
+`Limits::default()` requests desktop-tier compute-shader limits
+unconditionally (`max_compute_workgroups_per_dimension: 65535`) even
+though nothing in `iris`/`iris-core` uses a `ComputePipeline` — that
+crashed device creation outright on a downlevel GL adapter reporting
+OpenGL ES 3.0 (no compute shaders at all: the Android emulator's
+`EMU_GPU=software` path, and any real GLES-3.0-only Android device).
+`device_limits()` is what both `android::render::AndroidRenderer::new`
+and `default::render::UiRenderer::new` now build their `required_limits`
+from, so the request cannot drift between the two backends.
+
+Before: `Limits { max_buffer_size: 1 << 30, ..Default::default() }`
+inlined in each backend. After: `iris_core::device_limits()`, which is
+the same thing with the six `max_compute_*` fields additionally zeroed.
+A caller building its own `DeviceDescriptor` outside these two backends
+(there are none today, but a third platform backend would want this)
+should call `device_limits()` rather than reaching for
+`Limits::default()` directly, unless it genuinely adds a compute pass —
+in which case it wants the specific compute limits that pass needs, not
+the desktop-tier default for everything.
+
 ## 2026-09-05 (later the same day): `iris_core::FrameReport` (RUST.md's I5 box)
 
 New public type, `iris_core::FrameReport` (re-exported from `iris_core`'s