feat(llm): enrich polish prompts with directing heuristics for higher image/video quality

[Yuyyxz]

Summary

The three default polish prompts (DEFAULT_STORYBOARD_POLISH_PROMPT, DEFAULT_VIDEO_POLISH_PROMPT, DEFAULT_R2V_POLISH_PROMPT) currently give the LLM only high-level instructions ("describe motion + camera"). This leaves quality to the model's defaults, which often produces vague adjectives ("beautiful / stunning / masterpiece") and stacked actions that cause limb deformation.

This PR injects a set of directing heuristics into the prompts, so the LLM polishes every shot with concrete, model-friendly constraints.

What changed

All changes are in src/apps/comic_gen/llm.py (3 default prompt strings, +29/-1). No call-site logic changed — these prompts were already designed to be overridable.

1. DEFAULT_STORYBOARD_POLISH_PROMPT (image)

• Ban filler adjectives; require concrete visual facts (light direction/quality/highlight/shadow)
• Require lighting sculpting (source direction + quality + highlight + shadow placement)
• Require material-level detail density (skin pores, fabric folds, dust in light)
• One image one job; enforce style consistency

2. DEFAULT_VIDEO_POLISH_PROMPT (video)

• One primary action per shot (stacking actions = 之前oss可以不用配置的，现在要强制使用了嘛？ #1 cause of limb breakdown)
• Motion-amplitude tiers (slight/slowly/uniform/obvious; default slight/slowly; ban violent/fast)
• Body linkage (turn head → shoulder turn → hair follows)
• Body-part-level detail (fingertips, jawline, Adam's apple, eyelashes, breath rhythm)
• Order multi-person actions explicitly
• Dialogue mouth-motion cue

3. DEFAULT_R2V_POLISH_PROMPT (reference-to-video)

• Verbatim anchor restatement (appearance anchors must be copied verbatim, no drift)
• Motion-amplitude tiers
• Style consistency with reference assets

Verification

Tested against a deliberately bad draft ("a beautiful girl stands up, walks to the window, turns, sits, stands again, gorgeous, masterpiece, cinematic"). The polished output now includes motion-amplitude words (slowly/gently) and body linkage ("head rotates smoothly, causing her hair to sway; shoulders dip and rise") — heuristics the old prompt never produced.

Notes

• These are default prompts; per-project overrides via PromptConfig still take precedence (existing 3-level fallback unchanged).
• Content is original methodology, no copyright concerns.

Happy to adjust wording or split into separate PRs if preferred.