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.

Sources/Shared/UI/AttachmentPreviewView.swift

@@ -1,4 +1,5 @@
 import Foundation
+import os
 import SwiftUI
 
 #if os(iOS)
@@ -239,7 +240,13 @@ struct AttachmentPreviewView: View {
         thumbnailImages = thumbnailImages.filter { retainedIDs.contains($0.key) }
         for draft in drafts where thumbnailImages[draft.id] == nil {
             guard let data = draft.thumbnailData,
-                  let image = DraftThumbnailImage(data: data) else { continue }
+                  let image = DraftThumbnailImage(data: data) else {
+                if draft.kind == .video {
+                    Logger(subsystem: "Luma", category: "video-preview")
+                        .warning("video draft has no decodable thumbnail: \(draft.filename) thumbnailBytes=\(draft.thumbnailData?.count ?? 0)")
+                }
+                continue
+            }
             thumbnailImages[draft.id] = image
             await Task.yield()
         }