feat: rename 'reference path' to 'middle frame path'

Updates DEFAULTS, standard_keys, UI label, and timeline known_keys.
Adds _migrate_key_renames() called on load_json to auto-migrate
existing JSON files with the old key name.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tab_timeline_ng.py

@@ -618,7 +618,7 @@ def _render_preview_fields(item_data: dict):
     known_keys = {
         'sequence_number', 'general_prompt', 'general_negative', 'current_prompt', 'prompt',
         'negative', 'camera', 'flf', 'seed', 'resolutions',
-        'frame_to_skip', 'vace schedule', 'video file path',
+        'frame_to_skip', 'vace schedule', 'video file path', 'middle frame path',
     }
     # also skip lora keys
     custom_keys = [