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-2025-9878.md

@@ -0,0 +1,35 @@
+# Corn yield trial — Tyler Deal (Danvers, IL, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** Danvers, IL · Mclean County
+- **Cooperator:** Tyler Deal
+- **Company rep:** Rick Urish
+- **Planted:** 4/26/2025
+- **Harvested:** 9/25/2025
+- **Previous crop:** Soybeans
+- **Row width:** 30"
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Deal-Danvers-IL.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 84P78 | TRE RIB | 298.47 | 25.8 | - | 16.3 |
+| 2 | AgriGold | A644-64VT2 | VT2P RIB | 296.64 | 24.0 | - | 14.5 |
+| 3 | ProHarvest | 86P55 | TRE RIB | 295.17 | 24.3 | - | 13.0 |
+| 4 | Becks | 6492VT2 | VT2P RIB | 291.59 | 23.1 | - | 9.4 |
+| 5 | AgriGold | A642-32VT2 | VT2P RIB | 288.78 | 24.0 | - | 6.6 |
+| 6 | AgriGold | A642-05VT2 | VT2P RIB | 286.2 | 24.6 | - | 4.0 |
+| 7 | AgriGold | A645-71STX | SS RIB | 284.63 | 24.3 | - | 2.5 |
+| 8 | AgriGold | A640-94VT4 | VT4P RIB | 284.05 | 23.4 | - | 1.9 |
+| 9 | AgriGold | A645-55VT2 | VT2P RIB | 279.42 | 23.5 | - | -2.7 |
+| 10 | AgriGold | A642-18SSP | SSPRO | 277.96 | 23.4 | - | -4.2 |
+| 11 | ProHarvest | 80P75 | PCE | 275.42 | 20.8 | - | -6.7 |
+| 12 | ProHarvest | PH | 82P96PCE PCE | 273.61 | 21.5 | - | -8.5 |
+| 13 | AgriGold | A645-30VT2 | VT2P RIB | 272.94 | 24.7 | - | -9.2 |
+| 14 | ProHarvest | PH | 81P20PCE PCE | 264.86 | 21.5 | - | -17.3 |
+| 15 | AgriGold | A640-24 | CONV | 262.6 | 24.4 | - | -19.6 |