Add gated degrade example for realistic jsQR-parity benchmarking

Verified side-by-side: anyd matches jsQR (reference decoder) on curved
(cylinder half-arc to 0.9) + rotated + blurred + noisy + JPEG QRs at
~5px/module. The earlier real-photo failures were degraded screenshot
crops that jsQR/ZXing/rqrr/zbar also cannot read — not a decoder gap.

Author: MagicalTux

Cargo.toml

@@ -46,3 +46,7 @@ harness = false
 [[example]]
 name = "qrdebug"
 required-features = ["cli"]
+
+[[example]]
+name = "degrade"
+required-features = ["cli"]

examples/degrade.rs

@@ -0,0 +1,43 @@
+// Realistic-degradation harness: render a QR, warp/blur/noise it, write PNG.
+use anyd::codes::qr::{EcLevel, QrEncoder};
+use anyd::render::render;
+use anyd::traits::Encode;
+use anyd::transform::{self, Axis, Rng};
+
+fn main() {
+    let mut a = std::env::args().skip(1);
+    let out = a.next().expect("out.png");
+    let cyl: f32 = a.next().unwrap().parse().unwrap();
+    let rot: f32 = a.next().unwrap().parse().unwrap();
+    let blur: f32 = a.next().unwrap().parse().unwrap();
+    let noise: f32 = a.next().unwrap().parse().unwrap();
+    let scale: usize = a.next().map(|s| s.parse().unwrap()).unwrap_or(6);
+
+    let e = QrEncoder::new();
+    let sym = e.build_text("https://anyd.dev/x", EcLevel::M).unwrap();
+    let enc = e.encode(&sym).unwrap();
+    let mut img = render(&enc, scale);
+    if cyl != 0.0 {
+        img = transform::cylinder(&img, cyl, Axis::Vertical);
+    }
+    if rot != 0.0 {
+        img = transform::rotate(&img, rot);
+    }
+    if blur != 0.0 {
+        img = transform::gaussian_blur(&img, blur);
+    }
+    if noise != 0.0 {
+        let mut rng = Rng::new(42);
+        img = transform::add_noise(&img, noise, &mut rng);
+    }
+    let png = oxideav_png::PngImage {
+        width: img.width() as u32,
+        height: img.height() as u32,
+        pixel_format: oxideav_png::PngPixelFormat::Gray8,
+        stride: img.width(),
+        data: img.pixels().to_vec(),
+        palette: Vec::new(),
+    };
+    std::fs::write(&out, oxideav_png::encode_png_image(&png).unwrap()).unwrap();
+    println!("wrote {out} {}x{}", img.width(), img.height());
+}