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-2024-9494.md

@@ -0,0 +1,32 @@
+# Corn yield trial — Nick Wehr (Sigourney, IA, 2024)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2024
+- **Location:** Sigourney, IA · Keokuk County
+- **Cooperator:** Nick Wehr
+- **Company rep:** Jake Ekstrand
+- **Planted:** 4/15/2024
+- **Harvested:** 10/8/2024
+- **Previous crop:** Soybeans
+- **Row width:** 30
+- **Population:** 32,500 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2024/11/Corn-Nick-Wehr-Sigourney-IA-52591.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 85P58 | SSRIB | 263.96 | 15.7 | 62.2 | 9 |
+| 2 | ProHarvest | 83P66 | VT2PRIB | 263.12 | 16 | 57.7 | 8.2 |
+| 3 | ProHarvest | 81P20 | PCE | 262.45 | 14.7 | 56.5 | 7.5 |
+| 4 | ProHarvest | 83P33 | DGVT2PRIB | 256 | 15.3 | 59.6 | 1.1 |
+| 5 | ProHarvest | 8360 | VT2PRIB | 254.34 | 15.4 | 58.3 | -0.6 |
+| 6 | ProHarvest | 80P75 | PCE | 254.15 | 13.6 | 58.8 | -0.8 |
+| 7 | ProHarvest | 84P30 | AA | 253.5 | 17.3 | 61.1 | -1.4 |
+| 8 | ProHarvest | 79P54 | SSPRO | 247.05 | 15.4 | 55.7 | -7.9 |
+| 9 | ProHarvest | 8015 | VT2PRIB | 239.88 | 13.7 | 60.2 | -15.1 |