feat(dce): 新建 oxide_dce crate 骨架并接入 workspace

- workspace.members 在 oxide_cfg 后追加 crates/oxide_dce
- Cargo.toml 骨架照抄 oxide_cfg（oxide_ir + oxide_cfg 依赖，dev-deps parser/emit/vm）
- lib.rs 中文文档头 + pub fn dce 入口（空函数退化先例，非空路径 Task 2/3 骨架）

crates/oxide_dce/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "oxide_dce"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+rust-version.workspace = true
+description = "OxideJS - dead code elimination pass over IR (reachability + use-count iteration + mark-sweep)"
+
+[dependencies]
+oxide_ir = { path = "../oxide_ir" }
+oxide_cfg = { path = "../oxide_cfg" }
+
+[dev-dependencies]
+oxide_parser = { path = "../oxide_parser" }
+oxide_emit = { path = "../oxide_emit" }
+oxide_vm = { path = "../oxide_vm" }