feat(ai): add AIRuntime actor skeleton + TokenChunk

按 W2 plan Task 5 落地推理串行化骨架:
- TokenChunk: Sendable struct (text + decodeRate tok/s)
- AIRuntime: actor 单例
  - Status: notReady / loading / ready / error(msg)
  - prepare() async throws: 幂等加载,失败回滚 status
  - generate(prompt:maxTokens:) -> AsyncThrowingStream: 流式输出
    跨 actor 边界用 snapshot 模式捕获 self.status/llmSession
  - lastDecodeRate: 给 UI 顶部条 / Live Activity 取
- AIRuntimeError: LocalizedError, 三种 case

WIP: Build will fail until Task 6 lands LLMSession (intentional).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

体己/AI/TokenChunk.swift

@@ -0,0 +1,6 @@
+import Foundation
+
+struct TokenChunk: Sendable {
+    let text: String
+    let decodeRate: Double  // tokens / second, 估算值
+}