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-2024-9563.md

@@ -0,0 +1,37 @@
+# Corn yield trial — Tom & Bill Mcgrew (Prairie City, IL, 2024)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2024
+- **Location:** Prairie City, IL · McDonough County
+- **Cooperator:** Tom & Bill Mcgrew
+- **Company rep:** Eric Yoder
+- **Planted:** 4/24/2024
+- **Harvested:** 10/10/2024
+- **Previous crop:** Soybeans
+- **Row width:** 30
+- **Population:** 35,700 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Fungicide:** Yes
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2024/11/Corn-Tom-Bill-McGrew-Prairie-City-IL-61470.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 83P66 | VT2PRIB | 298.07 | 14.5 | 58.8 | 23.3 |
+| 2 | ProHarvest | 8360 | VT2PRIB | 286.57 | 13.9 | 60 | 11.8 |
+| 3 | ProHarvest | 78P23 | PCE | 284.93 | 13 | 58.9 | 10.2 |
+| 4 | ProHarvest | 83P17 | PCE | 280.11 | 14 | 58.7 | 5.4 |
+| 5 | ProHarvest | 80P75 | PCE | 280.1 | 13.1 | 56.8 | 5.4 |
+| 6 | ProHarvest | 84P78 | TRERIB | 278.67 | 13.1 | 58.9 | 3.9 |
+| 7 | ProHarvest | 79P87 | SSRIB | 278.1 | 12.2 | 59.1 | 3.4 |
+| 8 | ProHarvest | 84P78 | TRERIB | 274 | 14 | 58.6 | -0.7 |
+| 9 | ProHarvest | 79P54 | SSPRO | 266.78 | 12.3 | 60 | -8 |
+| 10 | ProHarvest | 81P20 | PCE | 266.08 | 13.1 | 56.9 | -8.7 |
+| 11 | ProHarvest | 79P49 | VT2PRIB | 265.56 | 12.4 | 61 | -9.2 |
+| 12 | ProHarvest | 82P68 | SSPRO | 261.64 | 12.6 | 60.3 | -13.1 |
+| 13 | ProHarvest | 81P65 | D | 250.93 | 13.1 | 59.8 | -23.8 |