Add 4 independent seed brands: Latham + Stine + 1st Choice + Burrus (+623 varieties) (#17)

Co-authored-by: claude <claude@jpaul.io>
Co-committed-by: claude <claude@jpaul.io>

corpus/latham/latham-l-2031-e3.md

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+# L 2031 E3
+
+- **Vendor:** Latham Hi-Tech Seeds (independent family-owned, Alexander, IA)
+- **Brand:** Latham Hi-Tech Seeds
+- **Crop:** Soybeans
+- **Maturity group:** 2.0
+- **Trait(s):** Enlist E3
+- **Source:** https://www.lathamseeds.com/products/l-2031-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)
+
+---
+
+_Peking source SCN variety with nice, upright lateral branches. Contains the Rps1k phytopthera gene and excellent field tolerance for planting early and on tighter soils. Very good IDC and SDS tolerances makes for a broadly adapted variety that moves south well. Unique looking leaf structure will make L 2031 E3 the talk of the town. This medium height plant handles stress very well and is consistent across all yield environments._
+
+---
+
+## Agronomic Characteristics
+
+- **Relative Maturity:** 2.0
+- **Plant Height:** Medium
+- **Plant Type:** Medium Bushy
+- **Flower Color:** Purple
+- **Pubescence:** Light Tawny
+- **Pod Color:** Tan
+- **Hilum Color:** Brown
+
+## Disease Ratings
+
+- **SCN Resistance:** Peking
+- **Iron Chlorosis:** 2.0
+- **Stress Tolerance:** 2.0
+- **Phytophthora Root Rot:** K, 2.0
+- **Brown Stem Rot:** 1.5
+- **White Mold:** 2.0
+- **Sudden Death:** 2.5
+- **Frog Eye:** 1.5