Add viewport zoom to the framing

A view rect that composes with the crop in the same normalised space:
nesting one rect in the other is a multiply, so the shader needs no second
rect and no extra uniform slot.

Zoom is explicitly not an edit. It is excluded from is_active, from the
structure hash, and from the sidecar, so a zoomed view exports exactly as an
unzoomed one does. is_neutral now asks the framing whether it *edits* rather
than whether it is active — otherwise merely zooming would mark a clean file
dirty.

Because the render target keeps its size while the sampled region shrinks,
zooming raises the resolution the pipeline works at rather than magnifying
already-rendered pixels, which is what makes 1:1 inspection show real detail.

Assisted-by: LLM

core/dr-pipeline/src/graph.rs

@@ -269,8 +269,12 @@ impl EditGraph {
     }
 
     /// Whether any operation, or the framing, currently changes the image.
+    ///
+    /// Asks the framing whether it *edits*, not whether it is active: zoom
+    /// makes the framing active without changing the image, and reporting a
+    /// merely-zoomed image as edited would mark a clean file dirty.
     pub fn is_neutral(&self) -> bool {
-        !self.ops.iter().any(|o| o.is_active()) && !self.framing.is_active()
+        !self.ops.iter().any(|o| o.is_active()) && !self.framing.edits_image()
     }
 
     /// Generate the fused shader for the current state.