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-2025-9914.md

@@ -0,0 +1,32 @@
+# Corn yield trial — Scott Rayhons (Garner, IA, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** Garner, IA · Hancock County
+- **Cooperator:** Scott Rayhons
+- **Company rep:** Ryan Frederich
+- **Planted:** 5/10/2025
+- **Harvested:** 10/6/2025
+- **Previous crop:** Soybeans
+- **Row width:** 30
+- **Population:** 35,000 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Rayhons-Garner-IA.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 74P10 | PCE | 226.28 | 18.3 | 54.5 | 57.6 |
+| 2 | ProHarvest | 72P32 | D | 220.77 | 17.1 | 56.2 | 52.1 |
+| 3 | ProHarvest | 71P50 | SSPRO | 196.45 | 17.2 | 52.9 | 27.7 |
+| 4 | Channel | 200-40 | VT4P | 180.04 | 17 | 2.6 | 11.3 |
+| 5 | ProHarvest | 79P15 | V | 162.8 | 20.7 | 51.6 | -5.9 |
+| 6 | ProHarvest | 68P77 | AA | 145.52 | 15.1 | 55.9 | -23.2 |
+| 7 | ProHarvest | 70P44 | DV | 133.59 | 16.9 | 53.9 | -35.1 |
+| 8 | ProHarvest | 62P52 | D | 130.51 | 14.7 | 53.5 | -38.2 |
+| 9 | ProHarvest | 75P85 | SSRIB | 122.4 | 21 | 52.7 | -46.3 |