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-2024-9517.md

@@ -0,0 +1,36 @@
+# Corn yield trial — Devin Winterroth (Kempton, IL, 2024)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2024
+- **Location:** Kempton, IL · Livingston County
+- **Cooperator:** Devin Winterroth
+- **Company rep:** Jake Wilken
+- **Planted:** 4/25/2024
+- **Harvested:** 9/23/2024
+- **Previous crop:** Soybeans
+- **Row width:** 30"
+- **Population:** 35,000 seeds/acre
+- **Tillage:** Conventional
+- **Fungicide:** Yes
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2024/11/Corn-Devin-Winteroth-Kempton-IL-60946.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | Becks | 6041 | Qrome | 278.09 | 17.7 | 57.6 | 24.8 |
+| 2 | LG | 67C27 | SS RIB | 269.49 | 17.1 | 61.3 | 16.2 |
+| 3 | NuTech | 7056 | V | 265.99 | 20.6 | 50.9 | 12.7 |
+| 4 | Becks | 6458 | SS RIB | 259.07 | 19.0 | 56.7 | 5.8 |
+| 5 | Channel | 215-42 | SSPRO | 256.79 | 18.0 | 57.2 | 3.5 |
+| 6 | ProHarvest | 83P48 | SSPRO | 252.98 | 20.5 | 53.6 | -0.3 |
+| 7 | ProHarvest | 79P84 | SSPRO | 251.77 | 21.9 | 59.3 | -1.5 |
+| 8 | Becks | 6064 | AM | 250.04 | 16.6 | 59.1 | -3.2 |
+| 9 | Becks | 6258 | VT4P RIB | 248.75 | 20.2 | 54.3 | -4.5 |
+| 10 | Channel | 214-78 | VT2P RIB | 247.32 | 18.1 | 56.5 | -6.0 |
+| 11 | Channel | 212-63 | VT4P RIB | 240.66 | 22.3 | 49.6 | -12.6 |
+| 12 | Becks | 6469 | VT2P RIB | 237.37 | 21.3 | 54.5 | -15.9 |
+| 13 | Becks | 6692 | SSPRO | 234.37 | 22.2 | 51.0 | -18.9 |