test: add locdebug example + real EAN-13 scene fixture

Real captured frame region (native res) where detect::locate fails: the
EAN-13 at (250,205)-(790,405) decodes fine when cropped, but locate()
swallows it into a Matrix (0,0)-(900,416) mega-region and emits no linear
candidate there. Fixture + examples/locdebug.rs pin this for the locator fix.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: MagicalTux

Cargo.toml

@@ -50,3 +50,7 @@ required-features = ["cli"]
 [[example]]
 name = "degrade"
 required-features = ["cli"]
+
+[[example]]
+name = "locdebug"
+required-features = ["cli"]

examples/locdebug.rs

@@ -0,0 +1,20 @@
+// Diagnostic: what does detect::locate find on a real photo?
+use anyd::detect::{locate, LocateOptions};
+fn main() {
+    let path = std::env::args().nth(1).expect("usage: locdebug <png>");
+    let rgba = oxideav_png::decode_png_to_rgba(&std::fs::read(&path).unwrap()).unwrap();
+    let (w, h) = (rgba.width as usize, rgba.height as usize);
+    let luma: Vec<u8> = rgba.data.chunks_exact(4)
+        .map(|p| ((p[0] as u32 * 299 + p[1] as u32 * 587 + p[2] as u32 * 114) / 1000) as u8).collect();
+    let frame = anyd::GrayFrame::new(&luma, w, h).unwrap();
+    let cands = locate(&frame, &LocateOptions::default());
+    println!("frame {w}x{h}: {} candidates", cands.len());
+    for c in &cands {
+        let cr = c.location.outline.corners;
+        let (xs, ys): (Vec<f32>, Vec<f32>) = (cr.iter().map(|p| p.x).collect(), cr.iter().map(|p| p.y).collect());
+        let (x0, y0) = (xs.iter().cloned().fold(f32::MAX, f32::min), ys.iter().cloned().fold(f32::MAX, f32::min));
+        let (x1, y1) = (xs.iter().cloned().fold(0.0, f32::max), ys.iter().cloned().fold(0.0, f32::max));
+        let fam = c.symbology.map(|s| format!("{:?}", s.dimension())).unwrap_or("?".into());
+        println!("  {fam:8} ({:.0},{:.0})-({:.0},{:.0})  {:.0}x{:.0}", x0, y0, x1, y1, x1 - x0, y1 - y0);
+    }
+}