Add 4 independent seed brands: Latham, Stine, 1st Choice, Burrus (+623 varieties)

Four independent regional brands across IA/IN/IL (variety-identity sources,
each parsed into structured characteristics_groups so ratings embed):

- latham (264: 155 corn + 109 soy) — Latham Hi-Tech Seeds, Alexander IA.
  WordPress REST enum (/wp-json/wp/v2/varieties) + /products/<slug>/ detail
  HTML. Scale 1-9 LOWER=better (reversed, like NK/AgriPro).
- stine (217: 58 corn + 159 soy) — Stine Seed, Adel IA (largest US
  independent). sitemap enum + /{crop}/traits/<slug>/<code>/ detail HTML.
  Corn 1-9 (9=best); soy qualitative.
- first_choice (78: 52 corn + 22 soy + 4 wheat) — 1st Choice Seeds,
  Rushville IN (employee-owned). Per-crop sitemap -> detail HTML. Scale
  0-10 higher=better. ~40 older corn pages thin at source; wheat
  identity-only.
- burrus (64: 38 corn + 26 soy) — Burrus Seed, Arenzville IL. Seedware
  JSON API. Scale 1-10 (10=best). Brands Burrus/Power Plus/DONMARIO.
  robots ai-train=no + named-bot blocks; operator opted in, scraper uses a
  non-blacklisted UA + honors Crawl-delay 10.

All 623 validated through rag.chunk.chunks_from_variety (0 errors; 6
identity-only pages from source gaps). No chunk.py change needed (identity
sources auto-route to chunks_from_variety).

Docs:
- sources.json: 4 entries + Hoegemeyer added to _excluded_sources. The
  Corteva ToU (shared across pioneer.com / hoegemeyer.com / therightseed.com
  / corteva.com + the Vylor spinoff) bans scrapers + competitive use, so the
  whole Corteva family is one excluded ToU domain.
- docs_mcp/lessons.md: rating-scales updated with all 4 directions +
  an explicit cross-vendor warning (Latham 1=best vs Stine/Burrus higher=best
  — never compare raw numbers without _scale_direction).
- README + CLAUDE corpus inventory: now 2,268 variety + 6,787 trial records.

CI rebuilds the index from the committed corpus.

corpus/latham/latham-lh-6306-pce.md

@@ -0,0 +1,37 @@
+# LH 6306 PCE
+
+- **Vendor:** Latham Hi-Tech Seeds (independent family-owned, Alexander, IA)
+- **Brand:** Latham Hi-Tech Seeds
+- **Crop:** Corn
+- **Relative maturity:** 113 days
+- **Trait(s):** Powercore Enlist Corn
+- **Source:** https://www.lathamseeds.com/products/lh-6306-pce/
+- **Rating scale:** numeric ratings ~1-9 where LOWER = BETTER (1 = best / most tolerant / most resistant); confirmed by cross-referencing Product Overview prose vs values (top-rated stalks/roots cluster 1.0-1.5, weak traits 3.0-3.5, none approach 9). Categorical values pass through verbatim (SCN source 'PI 88788', Phytophthora 'Rps 1k', Anthracnose 'ASR', 'Medium Tall', 'Flex'). NA/blank = not rated.
+- **Service area:** Latham dealer network — Upper Midwest (IA/MN/WI/IL/ND/SD/NE)
+
+---
+
+_This PowerCore® Enlist® (PCE) Refuge Advanced (RA) is an exciting performer at 113-days relative maturity. You can use LH 6306 PCE RA in zone and south. Expect long, consistent ears with very large kernels and very good husk coverage. LH 6306 PCE RA is a Grain First / Silage Capable hybrid and is one of our preferred choices for tar spot._
+
+---
+
+## Agronomic Characteristics
+
+- **Early Vigor:** 2.0
+- **Stalk Strength:** 2.0
+- **Root Strength:** 3.0
+- **Stay Green:** 2.0
+- **Drydown:** 3.0
+- **Test Weight:** 2.0
+- **Drought Tolerance:** 2.0
+- **Foliar Fungicide:** 2.0
+- **Corn-on-Corn:** 2.0
+- **Ear Height:** Medium HIgh
+- **Ear Type:** Flex
+
+## Disease Ratings
+
+- **Goss’s Wilt:** 2.0
+- **Northern Leaf Blight:** 3.0
+- **Anthracnose Stalk Rot:** 2.0
+- **Gray Leaf Spot:** 2.0