Sharpen the capture with a separable unsharp mask

Capture sharpening as a two-pass unsharp mask in the detail stage: blur
along x, then along y, each pass applying a one-dimensional high-pass to
luminance so the composite preserves a flat field exactly and matches the
textbook kernel on any locally one-dimensional edge.

The radius is stated in source pixels and converted once per render, so a
radius tuned on a fit view is the radius the exported file gets. Below one
render pixel the operation declines to draw rather than showing sharpening
the file will not contain, and emits a single pass-through that still
carries the output transform.

The develop session now renders through render_detailed, which is what
lets an active neighbourhood operation reach the screen at all.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-gpu/tests/capture_sharpen.rs

@@ -217,10 +217,12 @@ fn a_proxy_and_an_export_sharpen_the_same_photograph() {
     let Some(ctx) = ctx() else { return };
     const SOURCE: u32 = 128;
     let source = step_edge(&ctx, SOURCE, 90, 150);
-    // Four source pixels, so that even the half-size proxy has a two-pixel
-    // sigma and resolves the radius — the honest cut-off is tested in
+    // The widest radius the slider offers, so that even the half-size proxy
+    // has a 1.5-pixel sigma and resolves it — the honest cut-off is tested in
     // `dr-pipeline`, and this test is about the case where both renders draw.
-    let graph = sharpened(100.0, 4.0, 0.0);
+    // Asking for more would be asking for a photograph nobody can produce:
+    // `EditGraph::set_param` clamps to the descriptor on the way in.
+    let graph = sharpened(100.0, 3.0, 0.0);
 
     // The halo, measured against the same edit with no sharpening at the same
     // size: how far from the transition the picture is still disturbed, as a