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-soybeans-2025-9943.md

@@ -0,0 +1,33 @@
+# Soybean yield trial — Delturn (Monticello, IA, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Soybean
+- **Year:** 2025
+- **Location:** Monticello, IA · Jones County
+- **Cooperator:** Delturn
+- **Company rep:** Ryan Frederich
+- **Planted:** 4/29/2025
+- **Harvested:** 10/3/2025
+- **Previous crop:** Corn
+- **Row width:** 30
+- **Population:** 140,000 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Soy-Delturn-Monticello-IA.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | Pioneer | P28Z30E | - | 82.06 | 10.6 | 59.8 | 5.5 |
+| 2 | Apex | AE2860 | - | 81.86 | 10 | 59.7 | 5.3 |
+| 3 | Merschman | 2526E | - | 79.82 | 10.1 | 59.2 | 3.3 |
+| 4 | Apex | AE2560 | - | 77.57 | 10.2 | 58.9 | 1 |
+| 5 | Apex | AE2361 | - | 77.31 | 10.5 | 59 | 0.7 |
+| 6 | ProHarvest | 26E68 | - | 76.82 | 10.2 | 60.4 | 0.3 |
+| 7 | Apex | AE3150 | - | 76.09 | 10.6 | 59.7 | -0.5 |
+| 8 | ProHarvest | 25E35 | - | 74.48 | 10.3 | 59 | -2.1 |
+| 9 | Apex | AE2950 | - | 72.95 | 10.8 | 60.6 | -3.6 |
+| 10 | ProHarvest | 31E65S | - | 66.69 | 10.2 | 60.6 | -9.9 |