feat: Logo 嵌入 — QR 码中央叠加自定义图片

- PNG 渲染：RgbaImage 上使用 imageops::overlay 叠加 logo
- SVG 渲染：base64 编码 logo 嵌入 <image> 标签
- QrCode::to_png_bytes / to_svg 新增 Option<logo_bytes> 参数
- Logo 默认占 QR 区域 25%，建议配合 H 级纠错使用

core/src/render/svg.rs

@@ -1,6 +1,6 @@
 use crate::qr::QrCode;
 
-pub fn render_svg(qr: &QrCode) -> String {
+pub fn render_svg(qr: &QrCode, logo: Option<&[u8]>) -> String {
     let matrix_size = qr.size() as u32;
     let margin = qr.margin as u32;
     let total = matrix_size + 2 * margin;
@@ -20,9 +20,9 @@ pub fn render_svg(qr: &QrCode) -> String {
         .flat_map(|row| row.iter())
         .filter(|&&m| m)
         .count();
-    let mut svg = String::with_capacity(200 + dark_count * 50);
+    let mut svg = String::with_capacity(300 + dark_count * 50);
     svg.push_str(&format!(
-        r#"<svg xmlns="http://www.w3.org/2000/svg" width="{total}" height="{total}" viewBox="0 0 {total} {total}">"#
+        r#"<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="{total}" height="{total}" viewBox="0 0 {total} {total}">"#
     ));
     svg.push_str(&format!(
         r#"<rect width="{total}" height="{total}" fill="{bg}"/>"#
@@ -40,6 +40,42 @@ pub fn render_svg(qr: &QrCode) -> String {
         }
     }
 
+    // Logo 嵌入：base64 PNG data URL
+    if let Some(logo_bytes) = logo {
+        let b64 = base64_encode(logo_bytes);
+        let logo_size = total as f32 * 0.25;
+        let logo_x = (total as f32 - logo_size) / 2.0;
+        let logo_y = (total as f32 - logo_size) / 2.0;
+        svg.push_str(&format!(
+            r#"<image x="{logo_x}" y="{logo_y}" width="{logo_size}" height="{logo_size}" xlink:href="data:image/png;base64,{b64}"/>"#
+        ));
+    }
+
     svg.push_str("</svg>");
     svg
 }
+
+/// 简易 base64 编码（无外部依赖）
+fn base64_encode(data: &[u8]) -> String {
+    const CHARS: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/";
+    let mut result = String::with_capacity((data.len() + 2) / 3 * 4);
+    for chunk in data.chunks(3) {
+        let b0 = chunk[0] as u32;
+        let b1 = if chunk.len() > 1 { chunk[1] as u32 } else { 0 };
+        let b2 = if chunk.len() > 2 { chunk[2] as u32 } else { 0 };
+        let triple = (b0 << 16) | (b1 << 8) | b2;
+        result.push(CHARS[((triple >> 18) & 0x3F) as usize] as char);
+        result.push(CHARS[((triple >> 12) & 0x3F) as usize] as char);
+        if chunk.len() > 1 {
+            result.push(CHARS[((triple >> 6) & 0x3F) as usize] as char);
+        } else {
+            result.push('=');
+        }
+        if chunk.len() > 2 {
+            result.push(CHARS[(triple & 0x3F) as usize] as char);
+        } else {
+            result.push('=');
+        }
+    }
+    result
+}