feat: 格式扩展 — 支持 BMP/JPEG/WebP 输出

- png.rs 重命名为 image.rs，新增 OutputFormat 枚举
- QrCode::to_image_bytes 支持 PNG/BMP/JPEG/WebP
- CLI 新增 -f/--format 参数（png/bmp/jpeg/webp）
- Web API fmt 参数扩展至全部 4 种图像格式
- core/Cargo.toml: image crate 新增 bmp feature

core/src/render/image.rs

@@ -0,0 +1,148 @@
+//! QR 码图像渲染（支持 PNG/BMP/JPEG/WebP）
+//!
+//! 使用 `image` crate 将 QR 模块矩阵渲染为像素缓冲区，可选叠加 Logo。
+
+use crate::qr::QrCode;
+use image::{imageops, ImageBuffer, Rgba, RgbaImage};
+
+/// 输出图像格式
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum OutputFormat {
+    Png,
+    Bmp,
+    Jpeg,
+    WebP,
+}
+
+impl OutputFormat {
+    /// 转为 `image` crate 的格式枚举
+    fn to_image_format(self) -> image::ImageFormat {
+        match self {
+            Self::Png => image::ImageFormat::Png,
+            Self::Bmp => image::ImageFormat::Bmp,
+            Self::Jpeg => image::ImageFormat::Jpeg,
+            Self::WebP => image::ImageFormat::WebP,
+        }
+    }
+
+    /// 文件扩展名（不含点）
+    pub fn extension(self) -> &'static str {
+        match self {
+            Self::Png => "png",
+            Self::Bmp => "bmp",
+            Self::Jpeg => "jpeg",
+            Self::WebP => "webp",
+        }
+    }
+
+    /// MIME 类型
+    pub fn mime(self) -> &'static str {
+        match self {
+            Self::Png => "image/png",
+            Self::Bmp => "image/bmp",
+            Self::Jpeg => "image/jpeg",
+            Self::WebP => "image/webp",
+        }
+    }
+
+    /// 从扩展名解析
+    pub fn from_ext(ext: &str) -> Option<Self> {
+        match ext.to_lowercase().as_str() {
+            "png" => Some(Self::Png),
+            "bmp" => Some(Self::Bmp),
+            "jpeg" | "jpg" => Some(Self::Jpeg),
+            "webp" => Some(Self::WebP),
+            _ => None,
+        }
+    }
+}
+
+fn fill_module(
+    img: &mut RgbaImage,
+    x: u32,
+    y: u32,
+    module_size: u32,
+    is_dark: bool,
+    fg: &[u8; 3],
+    bg: &[u8; 3],
+) {
+    let color = if is_dark {
+        Rgba([fg[0], fg[1], fg[2], 255])
+    } else {
+        Rgba([bg[0], bg[1], bg[2], 255])
+    };
+    let x0 = x * module_size;
+    let y0 = y * module_size;
+    for dy in 0..module_size {
+        for dx in 0..module_size {
+            img.put_pixel(x0 + dx, y0 + dy, color);
+        }
+    }
+}
+
+fn overlay_logo(img: &mut RgbaImage, logo_bytes: &[u8], logo_size_pct: f32) -> Result<(), String> {
+    let logo = image::load_from_memory(logo_bytes).map_err(|e| format!("Logo 加载失败: {e}"))?;
+    let logo = logo.to_rgba8();
+    let img_w = img.width();
+    let img_h = img.height();
+    let logo_size = (img_w.min(img_h) as f32 * logo_size_pct) as u32;
+    if logo_size < 4 {
+        return Ok(());
+    }
+    let resized = imageops::resize(&logo, logo_size, logo_size, imageops::FilterType::Lanczos3);
+    let x = (img_w - logo_size) / 2;
+    let y = (img_h - logo_size) / 2;
+    imageops::overlay(img, &resized, x as i64, y as i64);
+    Ok(())
+}
+
+/// 渲染 QR 码到图像字节（支持 PNG/BMP/JPEG/WebP）
+pub fn render_image(
+    qr: &QrCode,
+    module_size: u8,
+    format: OutputFormat,
+    logo: Option<&[u8]>,
+) -> Result<Vec<u8>, image::ImageError> {
+    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 is_dark = if x >= margin
+                && x < margin + matrix_size
+                && y >= margin
+                && y < margin + matrix_size
+            {
+                let mx = (x - margin) as usize;
+                let my = (y - margin) as usize;
+                qr.modules()[my][mx]
+            } else {
+                false
+            };
+            fill_module(
+                &mut img,
+                x,
+                y,
+                module_size as u32,
+                is_dark,
+                &qr.fg_color,
+                &qr.bg_color,
+            );
+        }
+    }
+
+    if let Some(logo_data) = logo {
+        let _ = overlay_logo(&mut img, logo_data, 0.25);
+    }
+
+    let mut buf = Vec::new();
+    img.write_to(
+        &mut std::io::Cursor::new(&mut buf),
+        format.to_image_format(),
+    )?;
+    Ok(buf)
+}