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-2025-9896.md

@@ -0,0 +1,46 @@
+# Corn yield trial — Brett Halvorson (Northwood, IA, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** Northwood, IA · Worth County
+- **Cooperator:** Brett Halvorson
+- **Company rep:** Ryan Frederich
+- **Planted:** 5/10/2025
+- **Harvested:** 10/12/2025
+- **Row width:** 30
+- **Population:** 35,000 seeds/acre
+- **Tillage:** Strip/Zone
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Halvorson-Northwood-IA.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 79P54 | SSPRO | 229.61 | 20.4 | 56.8 | 24.3 |
+| 2 | ProHarvest | 76P42 | PCE | 227.68 | 17 | 54.2 | 22.4 |
+| 3 | ProHarvest | 71P50 | SSPRO | 226.9 | 18.1 | 55.1 | 21.6 |
+| 4 | ProHarvest | 78P95 | PCE | 224.12 | 20.8 | 55.2 | 18.8 |
+| 5 | ProHarvest | 80P75 | PCE | 223.83 | 21.6 | 52.9 | 18.5 |
+| 6 | ProHarvest | 79P15 | V | 223.01 | 21 | 53.8 | 17.7 |
+| 7 | ProHarvest | 76P42 | PCE | 222.56 | 17 | 53.6 | 17.3 |
+| 8 | ProHarvest | 81P20 | PCE | 215.36 | 24.2 | 50.9 | 10.1 |
+| 9 | Wyffels | W5080 | - | 215.29 | 17.1 | 54.7 | 10 |
+| 10 | ProHarvest | 76P42 | CONV | 214.04 | 17 | 55 | 8.8 |
+| 11 | ProHarvest | 78P95 | PCE | 210.39 | 21 | 55.9 | 5.1 |
+| 12 | ProHarvest | 73P40 | CONV | 207.95 | 16.4 | 56.3 | 2.7 |
+| 13 | ProHarvest | 81P19 | VT4PRIB | 207.09 | 22.9 | 53.3 | 1.8 |
+| 14 | ProHarvest | 75P85 | DGVT2PRIB | 203.16 | 16.8 | 54.9 | -2.1 |
+| 15 | ProHarvest | 74P10 | CONV | 201.54 | 17 | 56 | -3.7 |
+| 16 | ProHarvest | 77P25 | VT4PRIB | 200.45 | 16.9 | 54.6 | -4.8 |
+| 17 | ProHarvest | 81P65 | V | 198.48 | 20.5 | 53.2 | -6.8 |
+| 18 | ProHarvest | 72P32 | D | 196.99 | 17 | 57.7 | -8.3 |
+| 19 | ProHarvest | 74P10 | PCE | 193.84 | 16.4 | 57 | -11.4 |
+| 20 | ProHarvest | 75P85 | SSRIB | 189.29 | 18.2 | 55.4 | -16 |
+| 21 | ProHarvest | 71P16 | CONV | 188.98 | 14.1 | 55.1 | -16.3 |
+| 22 | Kluver | RWK | 07-201 | 180.38 | 16 | 58 | -24.9 |
+| 23 | Kluver | RWK | 103-170 | 173.86 | 16.3 | 57 | -31.4 |
+| 24 | Kluver | RWK | 96-490 | 151.93 | 15.1 | 56.9 | -53.4 |