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

- codex/quality.py: 3-signal filter (length / alpha-ratio / bib-score)
  + rule-based section classifier + run_quality_pass retroactive DB pass
- codex/ingest.py: promote quality imports to module level; apply
  filter_chunks before embedding; store section column in chunks INSERT
- codex/config.py: CHUNK_MIN_CHARS / CHUNK_MIN_ALPHA_RATIO / CHUNK_MAX_BIB_SCORE
- infra/schema.sql: ALTER TABLE chunks ADD COLUMN IF NOT EXISTS section TEXT
- .env.example: document F-16 quality thresholds
- codex/cli.py: quality run sub-command (scope by --paper-id or all papers)
- tests/quality/: 35 new tests covering all quality functions
- tests/ingest/: patch filter_chunks in source-path tests to isolate ingest

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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;