feat: 顶层 API + PNG/SVG/ASCII 渲染器

core/src/render/png.rs

@@ -1 +1,43 @@
-// FIXME: PNG 渲染器 — Task 10
+use crate::qr::QrCode;
+use image::{ImageBuffer, Luma};
+
+pub fn render_png(qr: &QrCode, module_size: u8) -> Vec<u8> {
+    let matrix_size = qr.size() as u32;
+    let margin = qr.margin as u32;
+    let total_size = matrix_size + 2 * margin;
+    let img_size = total_size * module_size as u32;
+
+    let mut img = ImageBuffer::new(img_size, img_size);
+
+    for y in 0..total_size {
+        for x in 0..total_size {
+            let module_x = x.saturating_sub(margin);
+            let module_y = y.saturating_sub(margin);
+
+            let is_dark = if module_x < matrix_size && module_y < matrix_size {
+                qr.modules()[module_y as usize][module_x as usize]
+            } else {
+                false // 白边
+            };
+
+            let px_val = if is_dark { 0u8 } else { 255u8 };
+            for dy in 0..module_size as u32 {
+                for dx in 0..module_size as u32 {
+                    img.put_pixel(
+                        x * module_size as u32 + dx,
+                        y * module_size as u32 + dy,
+                        Luma([px_val]),
+                    );
+                }
+            }
+        }
+    }
+
+    let mut buf = Vec::new();
+    img.write_to(
+        &mut std::io::Cursor::new(&mut buf),
+        image::ImageFormat::Png,
+    )
+    .expect("PNG 编码失败");
+    buf
+}