feat(AI): LLM 迁移到 mlx-swift-lm 2.31.3 + Qwen3.5-2B

将 SPM 依赖从 mlx-swift-examples 2.29.1 迁到改名延续仓库 mlx-swift-lm
2.31.3(含 qwen3_5 架构、旧 loadContainer API 兼容),文本 LLM 由
Qwen3-1.7B 换为 Qwen3.5-2B-4bit(走 qwen3_5→Qwen35Model 文本路径)。
连带 mlx-swift 0.29.1→0.31.4,顺修弃用 API:
- MLX.GPU.clearCache() → MLX.Memory.clearCache()
- MLX.GPU.set(cacheLimit:) → MLX.Memory.cacheLimit

更新 ModelManifest(.llm 文件清单+精确字节数,~1.63GiB)、ModelManifestTests、
HealthExport.modelTag 默认值。App BUILD SUCCEEDED + ModelManifestTests 通过。

保留作 MNN 改造的 GPU 兜底基线。

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

康康Tests/ModelManifestTests.swift

@@ -4,8 +4,8 @@ import Foundation
 
 struct ModelManifestTests {
 
-    @Test func llmHasNineFunctionalFiles() {
-        #expect(ModelManifest.files(for: .llm).count == 9)
+    @Test func llmHasTenFunctionalFiles() {
+        #expect(ModelManifest.files(for: .llm).count == 10)
     }
 
     @Test func vlHasFourteenFunctionalFiles() {
@@ -13,7 +13,7 @@ struct ModelManifestTests {
     }
 
     @Test func llmTotalBytesMatchesManifest() {
-        #expect(ModelManifest.totalBytes(for: .llm) == 984_013_244)
+        #expect(ModelManifest.totalBytes(for: .llm) == 1_749_079_691)
     }
 
     @Test func vlTotalBytesMatchesManifest() {
@@ -40,8 +40,8 @@ struct ModelManifestTests {
     }
 
     @Test func fileURLIsBaseSlashRepoSlashPath() {
-        let file = ModelFile(path: "config.json", bytes: 937)
+        let file = ModelFile(path: "config.json", bytes: 3_113)
         let url = ModelManifest.fileURL(for: .llm, file: file)
-        #expect(url.absoluteString == "https://file.myv0.com/Qwen3-1.7B-4bit/config.json")
+        #expect(url.absoluteString == "https://file.myv0.com/Qwen3.5-2B-4bit/config.json")
     }
 }