M12.2: merge main (ROI, viewer tools); gate GPU display textures

Region-of-interest frames render on the CPU path; the viewer keeps main's
frame drawing (Show Channel, exposure, ROI) and reads GPU-composited frames
back. Keeping frames as native egui-wgpu textures stays implemented behind
RenderSource::gpu_display (off).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/gpu/src/walk.rs

@@ -20,6 +20,10 @@ use crate::ops;
 /// `Renderer::comp_frame_cpu` returns it). `None` when something cannot run here (the caller
 /// renders on the CPU instead).
 pub(crate) fn render<'g>(e: &mut Enc<'g>, r: &Renderer, comp_id: ItemId, t: Tick) -> Option<GpuImage> {
+    // Region-of-interest frames are cropped on the CPU path.
+    if r.opts.roi.is_some() {
+        return None;
+    }
     let ctx = r.eval_ctx(comp_id, t)?;
     let s = r.opts.scale;
     let w = ((ctx.comp.width as f64 * s).round() as u32).max(1);