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/Cargo.toml

@@ -14,7 +14,7 @@ categories.workspace = true
 rust-version.workspace = true
 
 [dependencies]
-image = { version = "0.25", default-features = false, features = ["png", "jpeg", "webp"] }
+image = { version = "0.25", default-features = false, features = ["png", "jpeg", "webp", "bmp"] }
 serde = { version = "1", features = ["derive"] }
 
 [dev-dependencies]