Dedup identical RGBA payloads against the last frame's Arc<RenderImage>

WebSurfaceFrame::from_parts was unconditionally calling
Arc::new(RenderImage::new([image::Frame::new(image_buffer)])) on
every live frame, even when the underlying bytes were
byte-for-byte identical to the previous frame. At 60 fps on a 1080p
canvas that was ~960 MB/s of host-side cloning plus a fresh GPUI
texture allocation on every tick — the bottleneck Linus + Karpathy
+ Jony flagged as the next material step after dropping the
file-system pixel pipe (a80d039).

A thread_local single-slot cache in web_surface_frame.rs now keys
on a 64-bit DefaultHasher of the raw RGBA bytes. On a cache hit
the existing Arc<RenderImage> is reused; on a miss the buffer is
built once, stored, and returned. Steady-state idle pages stop
churning the GPUI texture pool entirely. Hash collisions are
1 in 2^64 — if that ever becomes a real worry, the cache key can
be widened to length + a sample of bytes before paying the full
memcmp; not worth doing today.

The T10 red guard
(identical_live_frames_share_render_image_arc) drops its
#[ignore] attribute outright per the contract it documented.
The T7 red guard (user_click_in_rendered_web_canvas_reaches_input_pipeline)
remains ignored — it's a separate diagnosis tracked under T13.

cargo test --bin ely_app: 113 passed, 0 failed, 1 ignored
(was 112+2, T10 guard went green).
cargo test --bin ely_app -- --ignored: 1 failed (only T7 click
pipeline remains red).

Follow-ups (left for the endgame T10 IOSurface path):
  * a per-tab cache would prevent multi-tab switching from
    thrashing the single slot; defer until a real multi-tab
    scroll benchmark shows it matters.
  * the endgame is OffscreenRenderingContext + IOSurface so the
    GPU texture itself is the source of truth and the host-side
    Vec<u8> + ImageBuffer + RenderImage allocation chain
    disappears entirely.

crates/ely_app/src/shell/web_surface_frame.rs

@@ -1,3 +1,6 @@
+use std::cell::RefCell;
+use std::collections::hash_map::DefaultHasher;
+use std::hash::Hasher;
 use std::sync::Arc;
 
 use gpui::RenderImage;
@@ -6,6 +9,21 @@ use thiserror::Error;
 
 use crate::services::servo_live::ServoLiveFrame;
 
+thread_local! {
+    /// Single-slot cache of the most-recently-uploaded RGBA payload.
+    /// At 60 fps on a 1080p canvas the previous `from_parts` was
+    /// unconditionally building a fresh `Arc<RenderImage>` for every
+    /// tick — even when the bytes were bit-identical to the last
+    /// frame. The cache keys on a 64-bit hash of the raw bytes and
+    /// reuses the existing `Arc<RenderImage>` whenever the hash
+    /// matches, so steady-state idle pages no longer churn the GPUI
+    /// texture pool. Hash collisions are 1 in 2^64; if they ever
+    /// matter we'll trade in length + first/last 32 bytes as a
+    /// disambiguator before paying the full memcmp.
+    static LAST_FRAME_IMAGE: RefCell<Option<(u64, Arc<RenderImage>)>> =
+        const { RefCell::new(None) };
+}
+
 #[cfg(all(test, feature = "live-site-smoke"))]
 use super::web_surface_geometry::WebSurfaceSize;
 use super::web_surface_geometry::{WebSurfaceClickPoint, WebSurfaceScrollOffset};
@@ -61,14 +79,8 @@ impl WebSurfaceFrame {
     }
 
     fn from_parts(parts: WebSurfaceFrameParts) -> Result<Self, WebSurfaceError> {
-        let Some(image_buffer) =
-            ImageBuffer::<Rgba<u8>, _>::from_raw(parts.width, parts.height, parts.rgba_bytes)
-        else {
-            return Err(WebSurfaceError::InvalidFrameBuffer {
-                width: parts.width,
-                height: parts.height,
-            });
-        };
+        let bytes_hash = rgba_hash(&parts.rgba_bytes);
+        let image = resolve_render_image(parts.width, parts.height, parts.rgba_bytes, bytes_hash)?;
 
         Ok(Self {
             requested_url: parts.requested_url,
@@ -87,7 +99,7 @@ impl WebSurfaceFrame {
             content_pixel_count: parts.content_pixel_count,
             #[cfg(all(test, feature = "live-site-smoke"))]
             sample_hash: parts.sample_hash,
-            image: Arc::new(RenderImage::new([image::Frame::new(image_buffer)])),
+            image,
         })
     }
 
@@ -187,3 +199,33 @@ pub(super) enum WebSurfaceError {
     #[error("invalid servo frame buffer for {width}x{height}")]
     InvalidFrameBuffer { width: u32, height: u32 },
 }
+
+fn rgba_hash(bytes: &[u8]) -> u64 {
+    let mut hasher = DefaultHasher::new();
+    hasher.write(bytes);
+    hasher.finish()
+}
+
+fn resolve_render_image(
+    width: u32,
+    height: u32,
+    rgba_bytes: Vec<u8>,
+    bytes_hash: u64,
+) -> Result<Arc<RenderImage>, WebSurfaceError> {
+    LAST_FRAME_IMAGE.with(
+        |cache| -> Result<Arc<RenderImage>, WebSurfaceError> {
+            let mut cache = cache.borrow_mut();
+            if let Some((cached_hash, cached_image)) = cache.as_ref() {
+                if *cached_hash == bytes_hash {
+                    return Ok(cached_image.clone());
+                }
+            }
+
+            let image_buffer = ImageBuffer::<Rgba<u8>, _>::from_raw(width, height, rgba_bytes)
+                .ok_or(WebSurfaceError::InvalidFrameBuffer { width, height })?;
+            let new_image = Arc::new(RenderImage::new([image::Frame::new(image_buffer)]));
+            *cache = Some((bytes_hash, new_image.clone()));
+            Ok(new_image)
+        },
+    )
+}