fix #5 camera video preview with settling and diagnostics

- Wait until the picker movie's file size stops changing before
  staging it, so a movie whose final bytes are still being flushed is
  never copied truncated (which produced drafts without a thumbnail
  and unplayable previews).
- Log the whole camera video path (pickup size, staging result,
  analyze outcome, thumbnail decode) under the video-preview os_log
  category for fast on-device diagnosis.
- Cover the video analysis pipeline with a simulator test that
  generates an HEVC QuickTime movie and asserts duration plus JPEG
  thumbnail, and guard the diagnostics in Scripts/verify.sh.

Tests/MediaPreviewProcessorVideoTests.swift

@@ -0,0 +1,68 @@
+import AVFoundation
+import XCTest
+@testable import Luma
+
+final class MediaPreviewProcessorVideoTests: XCTestCase {
+    /// Reproduces the camera-video analysis on the simulator: the pipeline
+    /// must produce a duration and a JPEG thumbnail for a freshly written
+    /// HEVC QuickTime movie.
+    func testVideoAnalysisProducesDurationAndThumbnail() async throws {
+        let url = try await Self.makeTestMovie()
+        defer { try? FileManager.default.removeItem(at: url) }
+
+        let processor = MediaPreviewProcessor()
+        let analysis = await processor.analyze(
+            url: url,
+            kind: .video,
+            mimeType: "video/quicktime"
+        )
+        XCTAssertNotNil(analysis.duration)
+        if let duration = analysis.duration {
+            XCTAssertGreaterThan(duration, 0)
+        }
+        XCTAssertNotNil(analysis.thumbnailData)
+        XCTAssertFalse(analysis.thumbnailData?.isEmpty ?? true)
+    }
+
+    private static func makeTestMovie() async throws -> URL {
+        let output = FileManager.default.temporaryDirectory
+            .appendingPathComponent("test-camera-\(UUID().uuidString).mov")
+        let writer = try AVAssetWriter(outputURL: output, fileType: .mov)
+        let input = AVAssetWriterInput(
+            mediaType: .video,
+            outputSettings: [
+                AVVideoCodecKey: AVVideoCodecType.hevc,
+                AVVideoWidthKey: 640,
+                AVVideoHeightKey: 480,
+            ]
+        )
+        input.expectsMediaDataInRealTime = true
+        let adaptor = AVAssetWriterInputPixelBufferAdaptor(
+            assetWriterInput: input,
+            sourcePixelBufferAttributes: [
+                kCVPixelBufferPixelFormatTypeKey as String: kCVPixelFormatType_32BGRA,
+                kCVPixelBufferWidthKey as String: 640,
+                kCVPixelBufferHeightKey as String: 480,
+            ]
+        )
+        writer.add(input)
+        guard writer.startWriting() else {
+            throw writer.error ?? NSError(domain: "test", code: 1)
+        }
+        writer.startSession(atSourceTime: .zero)
+        for frame in 0..<60 {
+            while !input.isReadyForMoreMediaData {
+                try await Task.sleep(nanoseconds: 2_000_000)
+            }
+            var buffer: CVPixelBuffer?
+            CVPixelBufferCreate(
+                kCFAllocatorDefault, 640, 480, kCVPixelFormatType_32BGRA, nil, &buffer
+            )
+            adaptor.append(buffer!, withPresentationTime: CMTime(value: Int64(frame), timescale: 30))
+        }
+        input.markAsFinished()
+        await writer.finishWriting()
+        return output
+    }
+}
+