feat(ai): add ModelStore with path management and bundle seed

按 W2 plan Task 4 落地模型路径管理:
- ModelKind enum: llm (Qwen3-1.7B-MLX-4bit) / vl (Qwen2.5-VL-3B-MLX-4bit)
- 用 config.json 作为 sentinel 判定模型是否就绪
- isReady / localURL / totalBytes 三个查询接口
- seedFromBundle(_:) 占位:Demo 现场预装模型旁路(W6 启用)
- shared 单例用 Application Support/Models/

测试 3 条:fresh / mark-ready / totalBytes,均用临时目录隔离 + defer cleanup。

注:.swift 文件需用户在 Xcode 拖入 target,⌘U 确认绿后 amend build commit。

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

体己/AI/ModelStore.swift

@@ -0,0 +1,76 @@
+import Foundation
+
+enum ModelKind: String, CaseIterable {
+    case llm = "Qwen3-1.7B-MLX-4bit"
+    case vl  = "Qwen2.5-VL-3B-MLX-4bit"
+
+    var displayName: String {
+        switch self {
+        case .llm: return "Qwen3-1.7B"
+        case .vl:  return "Qwen2.5-VL-3B"
+        }
+    }
+
+    /// 用于判定该模型是否已就绪的最小标志文件
+    var sentinelFilename: String { "config.json" }
+}
+
+final class ModelStore {
+    static let shared: ModelStore = {
+        do {
+            let appSupport = try FileManager.default.url(
+                for: .applicationSupportDirectory,
+                in: .userDomainMask,
+                appropriateFor: nil,
+                create: true
+            )
+            let root = appSupport.appendingPathComponent("Models", isDirectory: true)
+            return try ModelStore(rootURL: root)
+        } catch {
+            fatalError("ModelStore.shared init failed: \(error)")
+        }
+    }()
+
+    let rootURL: URL
+
+    init(rootURL: URL) throws {
+        self.rootURL = rootURL
+        try FileManager.default.createDirectory(at: rootURL, withIntermediateDirectories: true)
+    }
+
+    func localURL(for kind: ModelKind) -> URL {
+        rootURL.appendingPathComponent(kind.rawValue, isDirectory: true)
+    }
+
+    func isReady(_ kind: ModelKind) -> Bool {
+        let sentinel = localURL(for: kind).appendingPathComponent(kind.sentinelFilename)
+        return FileManager.default.fileExists(atPath: sentinel.path)
+    }
+
+    func totalBytes(for kind: ModelKind) -> Int {
+        let folder = localURL(for: kind)
+        guard let enumerator = FileManager.default.enumerator(
+            at: folder,
+            includingPropertiesForKeys: [.fileSizeKey]
+        ) else { return 0 }
+        var sum = 0
+        for case let url as URL in enumerator {
+            if let size = try? url.resourceValues(forKeys: [.fileSizeKey]).fileSize {
+                sum += size
+            }
+        }
+        return sum
+    }
+
+    /// Demo 现场旁路:从 Bundle 拷贝预装模型(W6 才真正使用,本周占位)
+    func seedFromBundle(_ kind: ModelKind) throws {
+        guard let bundleURL = Bundle.main.url(forResource: kind.rawValue, withExtension: nil) else {
+            return
+        }
+        let target = localURL(for: kind)
+        if FileManager.default.fileExists(atPath: target.path) {
+            try FileManager.default.removeItem(at: target)
+        }
+        try FileManager.default.copyItem(at: bundleURL, to: target)
+    }
+}