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-2025-9910.md

@@ -0,0 +1,42 @@
+# Corn yield trial — Kevin McMath (Clinton, IL, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** Clinton, IL · Dewitt County
+- **Cooperator:** Kevin McMath
+- **Company rep:** Rick Urish
+- **Planted:** 5/6/2025
+- **Harvested:** 10/6/2025
+- **Row width:** 30"
+- **Population:** 35,500 seeds/acre
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-McMath-Clinton-IL.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | Dekalb | 114-99 | VT4P RIB | 282.05 | 15.5 | 57.8 | 25.9 |
+| 2 | Pro | Harvest | 84P78TRE TRE RIB | 278.99 | 15.0 | 58.6 | 22.9 |
+| 3 | Burrus | 8K34VT2P | VT2P RIB | 276.76 | 15.2 | 59.1 | 20.6 |
+| 4 | Pioneer | P13777PCE | PCE | 265.91 | 15.4 | 59.6 | 9.8 |
+| 5 | Pro | Harvest | 86P55TRE TRE RIB | 263.77 | 17.3 | 57.4 | 7.7 |
+| 6 | Burrus-Power | Plus | 4V73AM AM | 261.83 | 12.3 | 56.9 | 5.7 |
+| 7 | Burrus-Power | Plus | 6G88VE VCE | 261.56 | 13.2 | 57.1 | 5.4 |
+| 8 | Burrus | 5C11PCE | PCE | 261.31 | 12.2 | 58.6 | 5.2 |
+| 9 | Burrus | 7F30 | CNV CONV | 258.45 | 13.7 | 59.8 | 2.3 |
+| 10 | Dekalbb | 66-18RIB | VT2P RIB | 258.43 | 14.5 | 55.2 | 2.3 |
+| 11 | Pioneer | 11616PCE | PCE | 256.29 | 12.8 | 57.6 | 0.2 |
+| 12 | Wyffels | W8086 | VT2P RIB | 255.33 | 13.4 | 57.8 | -0.8 |
+| 13 | Pioneer | P14830Q | Qrome | 254.45 | 13.7 | 57.7 | -1.7 |
+| 14 | Pro | Harvest | 80P75PCE PCE | 252.69 | 11.8 | 57.4 | -3.4 |
+| 15 | Wyffels | W7876 | VT2P RIB | 251.61 | 13.3 | 58.0 | -4.5 |
+| 16 | Wyffels | W7485RIB | TRE RIB | 246.66 | 12.2 | 59.2 | -9.5 |
+| 17 | Pro | Harvest | 81P20PCE PCE | 245.54 | 12.6 | 58.2 | -10.6 |
+| 18 | Pioneer | P1742Q | Qrome | 245.24 | 14.4 | 59.9 | -10.9 |
+| 19 | Dekalb | 66-06RIB | TRE RIB | 244.73 | 14.3 | 58.4 | -11.4 |
+| 20 | Burrus | 7B14PCE | PCE | 239.88 | 11.7 | 58.8 | -16.2 |
+| 21 | Dekalb | 110-41RIB | TRE RIB | 238.24 | 11.7 | 59.7 | -17.9 |
+| 22 | Pioneer | P10477V | VCE | 231.51 | 13.3 | 56.7 | -24.6 |