Add ProHarvest Seeds: 119 varieties + 161 cross-vendor plot reports

ProHarvest Seeds (independent Corn Belt brand, proharvestseeds.com) exposes
a public, no-auth WordPress REST API — cleaner ingestion than the HTML-only
independents. Two new sources:

- `proharvest` (variety identity, 119 row-crop varieties: 70 corn / 47 soy /
  2 wheat). Enumerated via /wp/v2/seed (seed-type taxonomy), agronomics
  parsed from each /seed/<slug>/ detail page into structured
  characteristics_groups so the ratings actually embed. Mixed scale: disease
  1-9 numeric (9=best, no flip), agronomic/general qualitative, soil HR/R.

- `proharvest_plots` (trials, data_type=trial, 161 plots, 2024+2025). Per-
  cooperator harvest reports via the custom /wp-json/proharvest/v1/plots?y=
  endpoint + PDF table extraction. Many are cross-vendor head-to-head
  (ProHarvest/Apex vs Pioneer/DEKALB/Becks/Channel/Wyffels). Handles ruled
  tables, unruled tables (text fallback; soy drops the Test-Wt column → 4 vs
  5 numerics), and off-template third-party reports (sanity-gated to verbatim
  so junk rows never ship). Image-only PDFs skipped + counted.

- rag/chunk.py: route proharvest_plots through the shared cross-vendor plot
  renderer (structured) / verbatim body (raw_text fallback).
- sources.json + lessons.md (rating-scales, trial-data).
- README/CLAUDE.md corpus inventory brought current (it had drifted: bayer
  931 not 475; ebberts/lg/agrigold were unlisted). New totals: 1,645 variety
  + 6,787 trial records.

robots.txt permissive (only search + /dealer-* disallowed); no ToS
automation clause. CI rebuilds the index from the committed corpus.

corpus/proharvest_plots/phpr-corn-2025-9928.md

@@ -0,0 +1,43 @@
+# Corn yield trial — Mike Van Zee (Sully, IA, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** Sully, IA · Jasper County
+- **Cooperator:** Mike Van Zee
+- **Company rep:** Jake Ekstrand
+- **Planted:** 4/12/2025
+- **Harvested:** 10/4/2025
+- **Previous crop:** Soybeans
+- **Row width:** 30
+- **Population:** 34,000 seeds/acre
+- **Tillage:** No-Till
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Van-Zee-Sully-IA.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 86P55 | TRERIB | 278.53 | 11.7 | 59 | 56.3 |
+| 2 | ProHarvest | 81P19 | VT4PRIB | 261.46 | 13.9 | 58 | 39.3 |
+| 3 | ProHarvest | 84P41 | PCE | 242.87 | 12.1 | 61 | 20.7 |
+| 4 | ProHarvest | 79P54 | SSPRO | 235.89 | 13.8 | 59 | 13.7 |
+| 5 | ProHarvest | 83P17 | PCE | 233.43 | 13.3 | 56 | 11.2 |
+| 6 | ProHarvest | 78P95 | PCE | 226.32 | 12.6 | 58 | 4.1 |
+| 7 | NuTech | 70F6 | V | 217.51 | 12 | 58 | -4.7 |
+| 8 | NuTech | 70B4 | AM | 216.03 | 12.6 | 58 | -6.2 |
+| 9 | NuTech | 72C1 | PCE | 215.35 | 14.9 | 58 | -6.8 |
+| 10 | ProHarvest | 83P66 | VT2PRIB | 215.32 | 13.3 | 54 | -6.9 |
+| 11 | ProHarvest | 82P96 | PCE | 214.99 | 12.5 | 56 | -7.2 |
+| 12 | NuTech | 71A7 | V | 214.68 | 12.1 | 58 | -7.5 |
+| 12 | NuTech | 73B2 | V | 214.68 | 12.1 | 58 | -7.5 |
+| 13 | NuTech | 73J1 | V | 214.43 | 12.2 | 57 | -7.8 |
+| 14 | ProHarvest | 81P45 | TRERIB | 214.42 | 10.7 | 62 | -7.8 |
+| 15 | ProHarvest | 85P58 | VT2PRIB | 212.76 | 11.5 | 60 | -9.4 |
+| 16 | NuTech | 71B6 | PCE | 212.17 | 12.6 | 58 | -10 |
+| 17 | ProHarvest | 81P20 | PCE | 206.42 | 12.2 | 55 | -15.8 |
+| 18 | NuTech | 69C7 | PCE | 199.39 | 12 | 58 | -22.8 |
+| 19 | NuTech | 69B5 | V | 197.24 | 11.8 | 59 | -25 |