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-2025-9886.md

@@ -0,0 +1,41 @@
+# Corn yield trial — Gary Erwin (Mcleansboro, IL, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** Mcleansboro, IL · Hamilton County
+- **Cooperator:** Gary Erwin
+- **Company rep:** Tom Forrest
+- **Planted:** 5/23/2025
+- **Harvested:** 10/14/2025
+- **Previous crop:** Soybeans
+- **Row width:** 30
+- **Population:** 30,000 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Fungicide:** Yes
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Erwin-Mcleansboro-IL.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 8360 | CONV | 221 | 17 | 58.6 | 12 |
+| 2 | Agrigold | A645-16 | CONV | 220.07 | 16.4 | 58.5 | 11.1 |
+| 3 | ProHarvest | 8360 | CONV | 219.16 | 15.1 | 59 | 10.2 |
+| 4 | Agrigold | A640-24 | CONV | 215.84 | 14.7 | 59.7 | 6.9 |
+| 5 | Pioneer | P17677 | CONV | 214.96 | 15.6 | 60.1 | 6 |
+| 6 | Pioneer | P1608 | CONV | 214.4 | 16 | 61.2 | 5.4 |
+| 7 | ProHarvest | X25725 | CONV | 212.61 | 15.7 | 58.1 | 3.6 |
+| 8 | ProHarvest | 83P66 | CONV | 212.07 | 16.1 | 58.1 | 3.1 |
+| 9 | Agrigold | A648-11 | CONV | 211.46 | 18.3 | 58.4 | 2.5 |
+| 10 | ProHarvest | 84P78 | CONV | 211.43 | 15.8 | 57.6 | 2.5 |
+| 11 | ProHarvest | 85P13 | CONV | 209.86 | 15.5 | 58 | 0.9 |
+| 12 | Agrigold | A646-30 | CONV | 206.09 | 15.8 | 59.1 | -2.9 |
+| 13 | ProHarvest | 8360 | CONV | 204.02 | 15.7 | 59.4 | -4.9 |
+| 14 | Golden Harvest | G11V76 | CONV | 203.58 | 15.3 | 58.2 | -5.4 |
+| 15 | Agrigold | A642-32 | CONV | 202.66 | 15 | 60.5 | -6.3 |
+| 16 | Agrigold | A638-19 | CONV | 192.18 | 16.3 | 58.6 | -16.8 |
+| 17 | Golden Harvest | G15J91 | CONV | 181.02 | 17.6 | 58.1 | -27.9 |