Generate a compact pool of discussion-worthy research directions from the signal table, writing `output/trace/IDEA_DIRECTION_POOL.md`. **Trigger**: idea direction pool, brainstorm directions, research directions, 研究方向池, brainstorm pool. **Use when**: you already have a signal table and want a small, non-isomorphic set of candidate directions. **Skip if**: `output/trace/IDEA_DIRECTION_POOL.md` already exists and is refined. **Network**: none. **Guardrail**: no invented papers; directions must stay anchored to the current signal table and core set.