iris: on-demand message-list/image benchmarks, and two O(N) findings

IRIS_TODO.md's "Benchmarks" item: a message list of N wrapped-text rows
(first-frame cost), scrolling it, and growing an input box above which
the list must move rather than re-layout -- all as a plain, harness=false
`cargo bench` binary (iris/benches/message_list.rs) since UiRenderState
touches no GPU or window, chosen over criterion because every scenario
here reduces to a count take_counters already answers exactly, and a
new dependency wasn't worth it. Scroll (200 ticks) and the input-grow
case (40 lines) are flat across N=100/1,000/10,000: LAYOUT.md's O(1)
move chain holds.

The many-images case (d) needs a real wgpu device, so it's a headless
example (iris/examples/bench_images.rs) plus a new
GpuTextures/UiRenderNode counter, take_image_bind_group_creates,
mirroring take_counters. It found two real non-O(1) costs, recorded as
new Fix items rather than redesigned: bind-group creation takes two
frames to settle after a cold load instead of one, and appending a
single image to an already-loaded 1,000-image list rebuilds all 1,000
existing bind groups (masks/move_offsets buffer growth triggers
rebuild_image_bind_groups unconditionally).

run-bench.sh wraps both. Numbers and commands are in IRIS_TODO.md.

cargo fmt --all -- --check, cargo clippy --all-targets, and
cargo test --workspace (19 passed) all clean; benches are not run by
cargo test.

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

iris/core/src/render/mod.rs

@@ -441,4 +441,11 @@ impl UiRenderNode {
     pub fn view_count(&self) -> usize {
         self.textures.view_count()
     }
+
+    /// Standalone-image bind groups built since the last call -- see
+    /// `GpuTextures::take_bind_group_creates`. Call once per frame before
+    /// `update()` to measure exactly that frame.
+    pub fn take_image_bind_group_creates(&mut self) -> u64 {
+        self.textures.take_bind_group_creates()
+    }
 }

iris/core/src/render/texture.rs

@@ -61,6 +61,14 @@ pub struct GpuTextures {
     /// nothing of its own to put there: rects and glyphs never sample it,
     /// but the layout requires something bound regardless.
     null_view: TextureView,
+
+    /// Standalone-image bind groups actually built (`create_image`'s own
+    /// build, or one per slot touched by `rebuild_image_bind_groups`) since
+    /// the last `take_bind_group_creates`. IRIS_TODO.md's "many images"
+    /// benchmark reads this to prove the steady-state cost of an
+    /// unchanging image list is zero, the same way `UiRenderState`'s
+    /// `draw_count`/`region_mut_count` prove the layout side.
+    bind_group_creates: u64,
 }
 
 impl GpuTextures {
@@ -297,12 +305,13 @@ impl GpuTextures {
                     masks,
                     move_offsets,
                 );
+                self.bind_group_creates += 1;
             }
         }
     }
 
     fn create_image(
-        &self,
+        &mut self,
         image: &DynamicImage,
         rsc_layout: &BindGroupLayout,
         masks: &ArrBuf<Mask>,
@@ -339,6 +348,7 @@ impl GpuTextures {
             masks,
             move_offsets,
         );
+        self.bind_group_creates += 1;
         ImageGpu {
             texture,
             view,
@@ -424,9 +434,17 @@ impl GpuTextures {
             page_count: 0,
             sampler,
             null_view,
+            bind_group_creates: 0,
         }
     }
 
+    /// Reads and zeroes the standalone-image bind-group creation counter --
+    /// call once per frame before `update()`, mirroring
+    /// `UiRenderState::take_counters`.
+    pub fn take_bind_group_creates(&mut self) -> u64 {
+        std::mem::take(&mut self.bind_group_creates)
+    }
+
     pub fn array_view(&self) -> &TextureView {
         &self.array_view
     }