Land a red TDD guard for live-frame texture re-upload (T10)

Every `WebSurfaceFrame::from_live_frame` today calls
`Arc::new(RenderImage::new([image::Frame::new(image_buffer)]))`
unconditionally — even when the underlying RGBA bytes are
byte-for-byte identical to the previous frame. At 60 fps on a 1080p
canvas that is roughly 960 MB/s of host-side cloning + a fresh
GPUI texture upload, and the roundtable agreed it is the next
material bottleneck after the file-system pipe (a80d039).

The contract this test pins is the cheapest invariant we can hold
against today's `SoftwareRenderingContext`: two `ServoLiveFrame`
inputs whose `rgba_bytes` are bit-identical must produce the same
underlying `Arc<RenderImage>` instance. Today they do not; the
ignored run confirms two distinct pointer values for back-to-back
identical inputs.

The fix has two recognised shapes. The interim shape lives entirely
in `WebSurfaceFrame::from_parts`: remember the previous frame's
bytes (hash or pointer-eq) and reuse the existing `Arc<RenderImage>`
when they match. The endgame shape removes the host-side image step
entirely — `OffscreenRenderingContext` + IOSurface — at which point
the assertion becomes meaningless and is replaced by a frame-time
budget. Whichever lands first, the fix commit MUST delete the
`#[ignore]` attribute outright; toggling its reason is a broken
contract.

To call `WebSurfaceFrame::from_live_frame` from a unit test without
spawning a real sidecar process, `ServoLiveFrame` gains a
`#[cfg(test)] pub(crate) fn for_test(...)` constructor that wraps
the existing private `from_parts` with realistic defaults. No
production path uses it.

cargo test --bin ely_app: 112 passed, 0 failed, 2 ignored.
cargo test --bin ely_app -- --ignored: 2 failed (expected RED:
T7 click pipeline + T10 texture re-upload).

crates/ely_app/src/services/servo_live.rs

@@ -236,6 +236,24 @@ impl ServoLiveFrame {
     pub fn into_rgba_bytes(self) -> Vec<u8> {
         self.rgba_bytes
     }
+
+    #[cfg(test)]
+    pub(crate) fn for_test(width: u32, height: u32, rgba_bytes: Vec<u8>) -> Self {
+        Self {
+            loaded_url: Some("https://example.com/".to_string()),
+            title: Some("Example".to_string()),
+            render_state: "complete".to_string(),
+            width,
+            height,
+            #[cfg(all(test, feature = "live-site-smoke"))]
+            non_white_pixel_count: 0,
+            #[cfg(all(test, feature = "live-site-smoke"))]
+            content_pixel_count: 0,
+            #[cfg(all(test, feature = "live-site-smoke"))]
+            sample_hash: 0,
+            rgba_bytes,
+        }
+    }
 }
 
 #[derive(Debug, Error)]