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-2024-9665.md

@@ -0,0 +1,40 @@
+# Corn yield trial — Todd Davis (Esbon, KS, 2024)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2024
+- **Location:** Esbon, KS · Jewell County
+- **Cooperator:** Todd Davis
+- **Company rep:** Derald Watson
+- **Planted:** 5/22/2024
+- **Harvested:** 10/23/2024
+- **Previous crop:** Soybeans
+- **Row width:** 30"
+- **Population:** 22,500 seeds/acre
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2024/11/Corn-Todd-Davis-Esbon-KS-66941.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | AgriGold | 638-19 | AA | 110.0 | 11.2 | 58.5 | 8.1 |
+| 2 | ProHarvest | 78P23 | PCE | 109.32 | 11.3 | 59.5 | 7.4 |
+| 3 | AgriGold | 636-39 | AA | 109.25 | 11.8 | 57.8 | 7.4 |
+| 4 | ProHarvest | 81P20 | PCE | 108.94 | 11.6 | 58.5 | 7.0 |
+| 5 | AgriGold | 643-52 | VT2P RIB | 108.73 | 12.7 | 60.2 | 6.8 |
+| 6 | Dekalb | DK62-89 | TRE RIB | 107.3 | 10.9 | 59.7 | 5.4 |
+| 7 | Dekalb | DK103-07 | TRE RIB | 104.24 | 11.7 | 58.3 | 2.3 |
+| 8 | AgriGold | 6572 | VT2P RIB | 103.44 | 12.9 | 60.7 | 1.5 |
+| 9 | Dekalb | DK61-41 | VT2P RIB | 103.31 | 11.8 | 57.3 | 1.4 |
+| 10 | Dekalb | DK62-22 | VT2P RIB | 103.22 | 11.5 | 59.4 | 1.3 |
+| 11 | ProHarvest | 8360 | VT2P RIB | 101.76 | 12.8 | 58.6 | -0.1 |
+| 12 | Channel | 214-78 | DGVT2P RIB | 99.39 | 12.4 | 59.2 | -2.5 |
+| 13 | Dekalb | DK62-70 | VT2P RIB | 96.54 | 11.7 | 58.9 | -5.4 |
+| 14 | ProHarvest | 83P33 | DGVT2P RIB | 96.31 | 12.3 | 58.0 | -5.6 |
+| 15 | AgriGold | 641-26 | V | 95.96 | 12.1 | 59.2 | -5.9 |
+| 16 | AgriGold | 643-24 | PCE | 94.98 | 13.0 | 60.0 | -6.9 |
+| 17 | Channel | 215-60 | TRE RIB | 94.86 | 12.6 | 57.8 | -7.0 |
+| 18 | Dekalb | DK105-35 | V | 86.6 | 11.0 | 58.3 | -15.3 |