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-2025-9828.md

@@ -0,0 +1,32 @@
+# Corn yield trial — Blair Hoerbert (San Jose, IL, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** San Jose, IL · Logan County
+- **Cooperator:** Blair Hoerbert
+- **Company rep:** Tom Forrest
+- **Planted:** 5/7/2025
+- **Harvested:** 9/26/2025
+- **Previous crop:** Corn
+- **Row width:** 30
+- **Population:** 36,000 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Fungicide:** Yes
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Hoerbert-San-Joes-IL-corn-on-corn-min.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 81P19 | VT4P RIB | 267.97 | 24.2 | 54.1 | 33.9 |
+| 2 | ProHarvest | 85P58 | SSRIB | 248.55 | 20.2 | 58.1 | 14.5 |
+| 3 | ProHarvest | 83P48 | SSPRO | 244.4 | 21.7 | 55.2 | 10.4 |
+| 4 | ProHarvest | 79P54 | SSPRO | 239.91 | 19.3 | 56.2 | 5.9 |
+| 5 | ProHarvest | 82P96 | PCE | 226.71 | 20.2 | 55.1 | -7.3 |
+| 6 | ProHarvest | 79P15 | V | 226.52 | 21 | 54.6 | -7.5 |
+| 7 | ProHarvest | 77P25 | TRERIB | 217.12 | 20.1 | 53 | -16.9 |
+| 8 | ProHarvest | 79P87 | SSRIB | 201.03 | 15.8 | 58.7 | -33 |