feat(AI): MNN 文本模型升到 Qwen3.5-4B(taobao-mnn 预转换)

现场机 iPhone 17(A19/SME2)内存与加速均可承载 4B,质量优于 2B。

- ModelKind.mnnLLM rawValue → "Qwen3.5-4B-MNN",displayName → Qwen3.5-4B (MNN/SME2)
- ModelManifest:7 个运行时文件(llm.mnn.weight ~2.45GB + 拆分的
  visual.mnn.weight 188MB),总计 2,836,770,850 bytes(~2.64GiB)
- ModelManifestTests:文件数 7 / 总字节 / URL 更新到 Qwen3.5-4B-MNN
- CLAUDE.md §2:MNN 主模型记为 Qwen3.5-4B,MLX 兜底仍 2B

模拟器 ModelManifestTests TEST SUCCEEDED。

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

康康Tests/ModelManifestTests.swift

@@ -20,12 +20,12 @@ struct ModelManifestTests {
         #expect(ModelManifest.totalBytes(for: .vl) == 3_109_729_929)
     }
 
-    @Test func mnnHasSixFunctionalFiles() {
-        #expect(ModelManifest.files(for: .mnnLLM).count == 6)
+    @Test func mnnHasSevenFunctionalFiles() {
+        #expect(ModelManifest.files(for: .mnnLLM).count == 7)
     }
 
     @Test func mnnTotalBytesMatchesManifest() {
-        #expect(ModelManifest.totalBytes(for: .mnnLLM) == 1_185_759_005)
+        #expect(ModelManifest.totalBytes(for: .mnnLLM) == 2_836_770_850)
     }
 
     @Test func mnnHasEssentialRuntimeFiles() {
@@ -39,7 +39,7 @@ struct ModelManifestTests {
     @Test func mnnFileURLUsesRepoPath() {
         let file = ModelFile(path: "config.json", bytes: 652)
         let url = ModelManifest.fileURL(for: .mnnLLM, file: file)
-        #expect(url.absoluteString == "https://file.myv0.com/Qwen3.5-2B-MNN/config.json")
+        #expect(url.absoluteString == "https://file.myv0.com/Qwen3.5-4B-MNN/config.json")
     }
 
     @Test func excludesReadmeAndGitattributes() {