feat(compiler): Compiler 增加 enable_dce 字段与 with_dce 开关（D-10）

- unit struct 改带字段 struct，new() 默认开启 DCE
- compile 在 emit 与 lower 之间插入 oxide_dce::dce（nested 不递归 D-03）
- Cargo.toml 追加 oxide_dce path 依赖；dev 环经 resolver v2 解析通过

crates/oxide_compiler/Cargo.toml

@@ -10,6 +10,7 @@ description = "OxideJS - AST to bytecode compiler"
 oxide_bytecode = { path = "../oxide_bytecode" }
 oxide_parser = { path = "../oxide_parser" }
 oxide_log = { path = "../oxide_log" }
+oxide_dce = { path = "../oxide_dce" }
 oxide_ir.workspace = true
 oxide_emit.workspace = true
 serde.workspace = true