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-2024-9557.md

@@ -0,0 +1,37 @@
+# Corn yield trial — Todd Arseneau (St. Anne, IL, 2024)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2024
+- **Location:** St. Anne, IL · Iroquois County
+- **Cooperator:** Todd Arseneau
+- **Company rep:** Jake Wilken
+- **Planted:** 5/23/2024
+- **Harvested:** 10/17/2024
+- **Previous crop:** Soybeans
+- **Row width:** 30"
+- **Population:** 32,000 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Fungicide:** Yes
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2024/11/Corn-Todd-Arseneau-St-Anne-IL-60954.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 83P33 | DGVT2P RIB | 234.37 | 18.2 | 57.7 | 17.8 |
+| 2 | Wyffels | W6215RIB | TRE RIB | 229.52 | 17.0 | 58.4 | 12.9 |
+| 3 | Wyffels | W8086RIB | VT2P RIB | 228.54 | 20.1 | 57.2 | 12.0 |
+| 4 | Wyffels | W6505 | TRE RIB | 225.55 | 17.3 | 58.5 | 9.0 |
+| 5 | Wyffels | W7945RIB | TRE RIB | 223.4 | 17.8 | 58.7 | 6.8 |
+| 6 | Wyffels | W6886RIB | VT2P RIB | 217.16 | 16.9 | 60.6 | 0.6 |
+| 7 | Wyffels | W6479 | SSPRO | 216.76 | 17.8 | 57.3 | 0.2 |
+| 8 | Wyffels | W7876RIB | VT2P RIB | 213.25 | 18.1 | 58.2 | -3.3 |
+| 9 | ProHarvest | 79P87 | SS RIB | 210.96 | 16.7 | 57.9 | -5.6 |
+| 10 | ProHarvest | 84P30 | AA | 210.53 | 21.3 | 57.1 | -6.0 |
+| 11 | ProHarvest | 83P48 | SSPRO | 209.06 | 20.0 | 59.2 | -7.5 |
+| 12 | ProHarvest | 81P20 | PCE | 202.99 | 18.0 | 56.7 | -13.6 |
+| 13 | Wyffels | W7759RIB | SSPRO | 193.35 | 19.1 | 56.8 | -23.2 |