epa_ppls: filter PPLS enumeration to row-crop products

The farmer-advisor consumer only cares about US row crops, so the EPA
scraper now drops products without at least one row-crop site in the
PPLS API response. Filter is on by default; --no-row-crop-filter
overrides for one-off broader pulls.

Filter shape:
  - Word-boundary regex match against each entry in the API's `sites`
    array (e.g., "SOYBEANS (FOLIAR TREATMENT)" → keep, "SHIPS, BOATS,
    SHIPHOLDS" → drop even though it contains "OATS" as substring).
  - Allowlist covers the major US row + small-grain + oilseed + sugar/
    fiber crops, plus alfalfa as a common rotation crop. See
    ROW_CROP_KEYWORDS in scrape/sources/epa_ppls.py for the full list.

Cost model:
  - 102K PPIS rows still need one API call each (no bulk filter
    available upstream), so enumeration still takes ~28h at 1 req/sec.
  - But PDF downloads drop from ~67K → ~5-10K (estimated row-crop
    hit rate), saving ~17h wall time and ~60GB disk on a full backfill.

Smoke test (4 mixed reg nos):
  524-475 Roundup Ultra        → kept (CORN/SOYBEANS/COTTON sites)
  524-591 Warrant              → kept (CORN/SOYBEANS/SORGHUM sites)
  100-1486 Advion Cockroach    → filtered (building/transport sites only)
  432-1276 (Bayer pet flea)    → filtered (no row crops)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

scrape/README.md

@@ -41,6 +41,27 @@ Every scraper is **idempotent** by default — re-running with the
 same arguments skips records already on disk. Use `--force` to
 re-fetch.
 
+## Scope: row crops only
+
+The corpus is scoped to **US row crops** — corn, soybeans, cotton,
+wheat, rice, sorghum/milo, barley, oats, rye, sunflowers, peanuts,
+sugar beets, dry/field beans, canola/rapeseed, and alfalfa. The
+EPA PPLS scraper enforces this by inspecting the `sites` array on
+each product's PPLS API response and dropping anything without a
+row-crop site (word-boundary match).
+
+The Bayer scraper doesn't filter — its catalog is implicitly
+ag-focused, and dropping fungicide/insecticide/seed-treatment
+products there would lose row-crop-relevant chemistry. Add
+per-source filters as needed if other manufacturer sources cover
+non-ag products.
+
+Override the EPA filter for a one-off broader pull:
+
+```bash
+python -m scrape.sources.epa_ppls --no-row-crop-filter --reg-no 100-1486
+```
+
 ## Canonical sidecar schema
 
 Every `corpus/<source>/<key>.json` conforms to this shape. Fields