feat(AI): 推理闸门双优先级 — 前台插队、后台按 token 让位;暴露统计与后端标签

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

康康Tests/InferencePriorityTests.swift

@@ -0,0 +1,28 @@
+import Testing
+@testable import 康康
+
+struct InferencePriorityTests {
+
+    @Test func interactiveJumpsAheadOfBackground() {
+        let idx = AIRuntime.gateInsertionIndex(of: .interactive,
+                                               in: [.interactive, .background, .background])
+        #expect(idx == 1)
+    }
+
+    @Test func interactiveKeepsFIFOAmongInteractive() {
+        let idx = AIRuntime.gateInsertionIndex(of: .interactive,
+                                               in: [.interactive, .interactive])
+        #expect(idx == 2)
+    }
+
+    @Test func backgroundAlwaysAppends() {
+        let idx = AIRuntime.gateInsertionIndex(of: .background,
+                                               in: [.interactive, .background])
+        #expect(idx == 2)
+    }
+
+    @Test func emptyQueueInsertsAtZero() {
+        #expect(AIRuntime.gateInsertionIndex(of: .interactive, in: []) == 0)
+        #expect(AIRuntime.gateInsertionIndex(of: .background, in: []) == 0)
+    }
+}