feat(desktop): P31 step4 story progress display + evidence shots

- Task panel (T key): a storyline row per occupation, e.g. ju-qing-xian:
shui-wei-yu-shou-cheng, zou-fang-zhong (line name + stage name); absent
for occupation-free saves.
- Occupation select page: minimal hint line showing the selected
  occupation's storyline name.
- Evidence hook LV_STORY_SHOT=1 (+LV_STORY_OCCUPATION, LV_STORY_STAGE):
  boots a real world through the standard bootstrap, advances the story
  via the real dialogue pipeline (chooseDialogueWith bias + Story.onDialogue
  + StoryAnnal append), opens the task panel and saves one back-buffer
  frame, then exits. 4 Xvfb shots (farmer/fisher/peddler/scholar) captured
  under art/ (gitignored, not committed).
- 4 new desktop tests (156 -> 160): panel storyline lines, bias + story
  advance through M5Interaction, no-occupation path stays story-free.

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