feat(F-16): chunk quality gate + section classification

3-signal quality filter (length/alpha-ratio/bib-score) + rule-based section classifier + retroactive CLI pass. 35 new tests, 287 total green.

infra/schema.sql

@@ -122,3 +122,6 @@ CREATE TABLE IF NOT EXISTS figures (
 CREATE INDEX IF NOT EXISTS figures_emb_idx
     ON figures USING hnsw (embedding vector_cosine_ops);
 CREATE INDEX IF NOT EXISTS figures_paper_idx ON figures (paper_id);
+
+-- F-16 Chunk Quality Gate: section classification
+ALTER TABLE chunks ADD COLUMN IF NOT EXISTS section TEXT;