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-l-2458-e3.md

@@ -0,0 +1,39 @@
+# L 2458 E3
+
+- **Vendor:** Latham Hi-Tech Seeds (independent family-owned, Alexander, IA)
+- **Brand:** Latham Hi-Tech Seeds
+- **Crop:** Soybeans
+- **Maturity group:** 2.4
+- **Trait(s):** Enlist E3
+- **Source:** https://www.lathamseeds.com/products/l-2458-e3/
+- **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)
+
+---
+
+_Outstanding yield potential is a key factor in this 2.4 maturity Enlist E3 soybean, for L 2458 E3, coming in at 105% of the overall mean in 2020 breeder trials. The defensive package is also good featuring features the Rps1-k gene for Phytophthora, SCN tolerance from PI 88788, full resistance to Brown Stem Rot and a very good score for Iron Chlorosis. Plant height is above average and the Stress tolerance is excellent allowing this soybean to be placed in both tough and high-yield all yield environments with great results! It is also tolerant to the Sulfonylurea (group 2) class of herbicides._
+
+---
+
+## Agronomic Characteristics
+
+- **Relative Maturity:** 2.4
+- **Emergence:** 1.7
+- **Plant Height:** Medium+
+- **Plant Type:** Medium
+- **Flower Color:** White
+- **Pubescence:** Gray
+- **Pod Color:** Tan
+- **Hilum Color:** Buff
+
+## Disease Ratings
+
+- **SCN Resistance:** F, 2.2
+- **Iron Chlorosis:** 2.3
+- **Stress Tolerance:** 1.5
+- **Phytophthora Root Rot:** K, 2.3
+- **Brown Stem Rot:** 1.0
+- **White Mold:** 2.8
+- **Sudden Death:** 2.5
+- **Charcoal Rot:** 2.0
+- **Frog Eye:** 2.5