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-2024-9647.md

@@ -0,0 +1,37 @@
+# Corn yield trial — Grow It to Show It (Delphos, OH, 2024)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2024
+- **Location:** Delphos, OH · Putnam County
+- **Cooperator:** Grow It to Show It
+- **Company rep:** Matthew Reichert
+- **Planted:** 5/8/2024
+- **Harvested:** 10/13/2024
+- **Row width:** 30"
+- **Population:** 34,500 seeds/acre
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2024/11/Corn-Grow-It-2-Show-It-Delphos-OH-45833.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | LG | Seeds | 59C72-5222EZ DV | 233.6 | 15.3 | 60.0 | 21.3 |
+| 2 | ProHarvest | 78P23 | PCE | 233.1 | 15.0 | 60.0 | 20.8 |
+| 3 | ProHarvest | 76P42 | PCE | 228.3 | 14.1 | 58.5 | 16.0 |
+| 4 | LG | Seeds | 61C34 STXRIB | 228.1 | 14.7 | 61.5 | 15.8 |
+| 5 | LG | Seeds | 60C86-5222EZ DV | 222.9 | 15.5 | 59.5 | 10.6 |
+| 6 | ProHarvest | 77P19 | VT2P RIB | 219.2 | 13.6 | 59.5 | 6.9 |
+| 7 | LG | Seeds | 58C73-3110 AA | 216.5 | 14.7 | 59.5 | 4.2 |
+| 8 | LG | Seeds | 58C48 VT2RIB | 214.4 | 14.4 | 59.25 | 2.1 |
+| 9 | ProHarvest | 75P85 | SS RIB | 211.7 | 13.7 | 60.0 | -0.6 |
+| 9 | LG | Seeds | 59C72-VT2RIB VT2P RIB | 211.7 | 14.8 | 59.25 | -0.6 |
+| 9 | ProHarvest | 80P75 | PCE | 211.7 | 14.0 | 59.0 | -0.6 |
+| 12 | LG | Seeds | 57C33 VT2RIB | 208.9 | 14.0 | 59.0 | -3.4 |
+| 13 | LG | Seeds | 53C44 VT2RIB | 208.7 | 14.0 | 62.5 | -3.6 |
+| 14 | ProHarvest | 81P20 | PCE | 200.6 | 14.3 | 59.5 | -11.7 |
+| 15 | LG | Seeds | 56C78-3110 AA | 197.8 | 14.2 | 59.0 | -14.5 |
+| 16 | ProHarvest | 73P40 | PCE | 149.7 | 14.3 | 61.0 | -62.6 |