Warm up AVFoundation encoder once before tests

The first AVAssetWriter/H.264 use on the CI simulator pays a large,
variable cold-start cost (30s+ observed), which landed inside the first
video test's timed expectation and could trip its timeout. Pay that cost
once in setUp, outside any XCTest expectation, so each test only measures
steady-state latency.

Author: claude

TimeLapseBuilder-Common/TimeLapseBuilderTests.swift

@@ -11,6 +11,40 @@ import XCTest
 @testable import TimeLapseBuilder
 
 class TimeLapseBuilderTests: XCTestCase {
+    private static var didWarmUpEncoder = false
+
+    override func setUp() {
+        super.setUp()
+        Self.warmUpEncoderIfNeeded()
+    }
+
+    // The first use of AVAssetWriter's H.264 encoder on the CI simulator pays a
+    // large and highly variable cold-start cost (30s+ has been observed). Pay it
+    // once here, before any test's timed expectation, so individual tests only
+    // measure steady-state encoding latency. This wait is not bounded by an
+    // XCTest expectation, so the cold start can't trip a test timeout.
+    private static func warmUpEncoderIfNeeded() {
+        guard !didWarmUpEncoder else { return }
+        didWarmUpEncoder = true
+
+        guard let warmUpAsset = Bundle(for: TimeLapseBuilderTests.self).url(forResource: "red", withExtension: "jpg")?.absoluteString else {
+            return
+        }
+
+        let semaphore = DispatchSemaphore(value: 0)
+        let delegate = TestDelegate(progress: { _ in }, finished: { _ in
+            semaphore.signal()
+        }, failed: { _ in
+            semaphore.signal()
+        })
+        let documentsPath = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0] as NSString
+        let outputPath = documentsPath.appendingPathComponent("WarmUp.mov")
+
+        let builder = TimeLapseBuilder(delegate: delegate)
+        builder.build(with: [warmUpAsset], atFrameRate: 30, type: .mov, toOutputPath: outputPath)
+        _ = semaphore.wait(timeout: .now() + 120)
+    }
+
     func testWhenGivenASeriesOfImages_producesAnOutputFile() {
         let expectation = self.expectation(description: "Build timelapse")
         let testDelegate = TestDelegate(progress: { (progress: Progress) in