Implement session cache upgrade phase 1 foundation

MLXServer/Server/CancellationToken.swift

@@ -0,0 +1,14 @@
+import os
+
+/// Thread-safe cancellation flag for cooperative stream shutdown.
+final class CancellationToken: @unchecked Sendable {
+    private let lock = OSAllocatedUnfairLock(initialState: false)
+
+    var isCancelled: Bool {
+        lock.withLock { $0 }
+    }
+
+    func cancel() {
+        lock.withLock { $0 = true }
+    }
+}