feat: define browser domain and application contracts

docs/api.md

@@ -4,33 +4,7 @@ MVP 首先提供本地 Go application service 和 CLI；是否增加 loopback HT
 
 ## 核心类型
 
-```go
-type BrowserKind string
-const (
-    BrowserChrome BrowserKind = "chrome"
-    BrowserEdge BrowserKind = "edge"
-)
-
-type LaunchSpec struct {
-    Kind BrowserKind
-    Executable string
-    UserDataDir string
-    ProfileDirectory string
-    TargetURL string
-    ProxyServer string
-    Headless bool
-    ExtraArgs []string
-}
-
-type InstanceView struct {
-    ID string
-    Kind BrowserKind
-    PID int
-    UserDataDir string
-    Status string
-    ExitCode *int
-}
-```
+代码权威类型位于 `internal/domain/browser.go`：`BrowserKind`、`LaunchSpec`、`InstanceStatus`、`InstanceView`、`BrowserEvent` 和稳定错误码。`LaunchSpec.Normalize()` 负责当前已确定的 browser kind、绝对 user data dir 和 http/https URL 校验。
 
 ## Application service