feat: QR 码解码器 — 从零手写的完整解码流水线

新增 core/src/decoder/ 模块（9 文件，~1500 行）：
- bch.rs: BCH(15,5)+BCH(18,6) 查表解码（32+64 有效码字，t≤3）
- format.rs: 从矩阵读取格式信息(EC+掩码)+版本信息（2 副本容错）
- extract.rs: 逆向蛇形排列提取数据码字
- deinterleave.rs: 逆向 RS 数据交错
- rs_decode.rs: RS 纠错流水线（伴随式→BM→Chien→Forney）
- mode_decode.rs: 逆向 4 种编码模式（数字/字母/字节/汉字 Shift JIS）
- detect.rs: 定位图案检测（1:1:3:1:1 比例+交叉验证+聚类）
- image.rs: 图像加载+灰度二值化（PNG/JPEG/WebP）
- mod.rs: 顶层 API（decode_image + decode_matrix）

修改已有文件：
- core: galois.rs 表 pub(crate), 新增 poly_eval(); reed_solomon 公开内部函数
- cli: 新增 --decode <file> 解码模式
- web: 新增 POST /api/decode（multipart file upload）

测试: 72 passed (58 原有 + 14 新增 decoder 测试)

Cargo.lock

@@ -172,6 +172,7 @@ dependencies = [
  "matchit",
  "memchr",
  "mime",
+ "multer",
  "percent-encoding",
  "pin-project-lite",
  "serde_core",
@@ -897,6 +898,15 @@ version = "1.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "4ef6b89e5b37196644d8796de5268852ff179b44e96276cf4290264843743bb7"
 
+[[package]]
+name = "encoding_rs"
+version = "0.8.35"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "75030f3c4f45dafd7586dd6780965a8c7e8e285a5ecb86713e63a79c5b2766f3"
+dependencies = [
+ "cfg-if",
+]
+
 [[package]]
 name = "equivalent"
 version = "1.0.2"
@@ -1717,10 +1727,23 @@ checksum = "85ab80394333c02fe689eaf900ab500fbd0c2213da414687ebf995a65d5a6104"
 dependencies = [
  "bytemuck",
  "byteorder-lite",
+ "image-webp",
  "moxcms",
  "num-traits",
  "png 0.18.1",
  "tiff",
+ "zune-core",
+ "zune-jpeg",
+]
+
+[[package]]
+name = "image-webp"
+version = "0.2.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "525e9ff3e1a4be2fbea1fdf0e98686a6d98b4d8f937e1bf7402245af1909e8c3"
+dependencies = [
+ "byteorder-lite",
+ "quick-error",
 ]
 
 [[package]]
@@ -2065,6 +2088,23 @@ dependencies = [
  "windows-sys 0.61.2",
 ]
 
+[[package]]
+name = "multer"
+version = "3.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "83e87776546dc87511aa5ee218730c92b666d7264ab6ed41f9d215af9cd5224b"
+dependencies = [
+ "bytes",
+ "encoding_rs",
+ "futures-util",
+ "http",
+ "httparse",
+ "memchr",
+ "mime",
+ "spin",
+ "version_check",
+]
+
 [[package]]
 name = "ndk"
 version = "0.9.0"
@@ -3272,6 +3312,12 @@ dependencies = [
  "system-deps",
 ]
 
+[[package]]
+name = "spin"
+version = "0.9.8"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "6980e8d7511241f8acf4aebddbb1ff938df5eebe98691418c4468d0b72a96a67"
+
 [[package]]
 name = "stable_deref_trait"
 version = "1.2.1"