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-2025-9870.md

@@ -0,0 +1,39 @@
+# Corn yield trial — Westin Arsenau (St. Anne, IL, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** St. Anne, IL · Iroquois County
+- **Cooperator:** Westin Arsenau
+- **Company rep:** Jake Wilken
+- **Planted:** 5/9/2025
+- **Harvested:** 10/14/2025
+- **Previous crop:** Soybeans
+- **Row width:** 30
+- **Tillage:** Strip/Zone
+- **Irrigation:** Non-Irrigated
+- **Fungicide:** Delaro Complete
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Arsenau-St-Anne-IL.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | Wyffels | W7876RIB | - | 255.52 | 22.8 | 55 | 15.8 |
+| 2 | Wyffels | W7945RIB | - | 251.45 | 21.7 | 55.2 | 11.8 |
+| 3 | ProHarvest | 85P58 | VT2PRIB | 248.27 | 19.7 | 59.7 | 8.6 |
+| 4 | ProHarvest | 84P41 | PCE | 244.77 | 20.3 | 55.5 | 5.1 |
+| 5 | Wyffels | W6505RIB | - | 244.5 | 20.4 | 57.5 | 4.8 |
+| 6 | Wyffels | W8086RIB | - | 244.46 | 21.3 | 55.7 | 4.8 |
+| 7 | ProHarvest | 86P55 | TRERIB | 242.92 | 21.6 | 57.9 | 3.2 |
+| 8 | ProHarvest | 80P75 | - | 242.65 | 18.6 | 54.9 | 3 |
+| 9 | Wyffels | 7424 | - | 241.95 | 20.5 | 56.6 | 2.3 |
+| 10 | ProHarvest | 82P96 | PCE | 238.71 | 20.3 | 55.5 | -1 |
+| 11 | Wyffels | W6886RIB | - | 238.09 | 20.1 | 57 | -1.6 |
+| 12 | ProHarvest | 81P45 | TRERIB | 235.6 | 20.6 | 55 | -4.1 |
+| 13 | Wyffels | W7485RIB | - | 235.17 | 20.9 | 54.9 | -4.5 |
+| 14 | Wyffels | 5684 | - | 231.55 | 18.4 | 57 | -8.1 |
+| 15 | ProHarvest | 78P95 | PCE | 226.49 | 18.9 | 59 | -13.2 |
+| 16 | Wyffels | 4515 | - | 212.94 | 18.4 | 55 | -26.8 |