feat: 集成 Obscura 进程内无头浏览器、极致编译瘦身 profile 与词典内存优化

- 下载器 Obscura 后备通道拆分为条件编译双路径：
    进程内模式 (obscura-inprocess feature) 通过 spawn_blocking + 单线程
    runtime 驱动 V8 直接抓取；默认外部命令行模式通过 bin/obscura 子进程调用
  - Cargo.toml 新增 obscura-browser/obscura-net 可选依赖与 release-min profile
    (LTO + strip + opt-level="s"，二进制 17→8.3 MB，VSZ 1.27G→302M)
  - 词典加载后 shrink_to_fit() 释放预留容量，降低常驻内存
  - README 与 deployment.md 扩写 Obscura 双模式部署及低配服务器优化指南
  - 新增 Obscura mock 集成测试，补齐测试 fixture 字段

Cargo.toml

@@ -45,3 +45,17 @@ flate2 = "1.1.9"
 zip = "8.6.0"
 uuid = { version = "1.23.2", features = ["v4"] }
 tracing-appender = "0.2.5"
+obscura-browser = { path = "libs/obscura/crates/obscura-browser", optional = true }
+obscura-net = { path = "libs/obscura/crates/obscura-net", optional = true }
+
+[features]
+default = []
+obscura-inprocess = ["dep:obscura-browser", "dep:obscura-net"]
+
+[profile.release-min]
+inherits = "release"
+opt-level = "s"
+lto = true
+codegen-units = 1
+strip = true
+