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-2024-9496.md

@@ -0,0 +1,44 @@
+# Corn yield trial — Delturn Inc (Monticello, IA, 2024)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2024
+- **Location:** Monticello, IA · Jones County
+- **Cooperator:** Delturn Inc
+- **Company rep:** Jake Ekstrand
+- **Planted:** 5/8/2024
+- **Harvested:** 10/22/2024
+- **Previous crop:** Corn
+- **Row width:** 30
+- **Population:** 36,000 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2024/11/Corn-Steve-Doug-Turnis-Monticello-IA-52310.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | Wyffels | W8086RIB | - | 320.84 | 14.8 | 60.7 | 34.9 |
+| 2 | ProHarvest | 85P13 | CONV | 312.37 | 15.4 | 61.4 | 26.4 |
+| 3 | ProHarvest | 84P78 | TRERIB | 305.27 | 14.4 | 62.2 | 19.3 |
+| 4 | ProHarvest | 83P48 | SSPRO | 304.83 | 16.2 | 61.2 | 18.9 |
+| 5 | ProHarvest | 82P68 | SSPRO | 304.77 | 15.3 | 62.7 | 18.8 |
+| 6 | Wyffels | W6886RIB | - | 304.03 | 14.3 | 62.6 | 18.1 |
+| 7 | Wyffels | W6479 | - | 303.65 | 15.3 | 57 | 17.7 |
+| 8 | ProHarvest | 81P20 | PCE | 299.61 | 15.9 | 58 | 13.7 |
+| 9 | ProHarvest | 80P75 | PCE | 298.43 | 13.6 | 60.6 | 12.5 |
+| 10 | ProHarvest | 78P23 | PCE | 295.23 | 14.9 | 59.4 | 9.3 |
+| 11 | Wyffels | W5406RIB | - | 287.17 | 13.7 | 61.1 | 1.2 |
+| 12 | ProHarvest | 79P87 | SSRIB | 280.52 | 14.1 | 62.3 | -5.4 |
+| 13 | ProHarvest | 8360 | VT2PRIB | 280.32 | 15.4 | 64 | -5.6 |
+| 14 | ProHarvest | 84P22 | DV | 279.89 | 16.7 | 59.2 | -6 |
+| 15 | ProHarvest | 84P30 | AA | 278.89 | 16.6 | 62.1 | -7 |
+| 16 | ProHarvest | 79P54 | SSPRO | 277.02 | 15.2 | 59.5 | -8.9 |
+| 17 | ProHarvest | 81P65 | D | 270.35 | 14.7 | 60.8 | -15.6 |
+| 18 | Wyffels | W7759RIB | - | 263.46 | 13.9 | 62.8 | -22.5 |
+| 19 | Wyffels | W5019RIB | - | 262.37 | 14.6 | 56.7 | -23.6 |
+| 20 | ProHarvest | 75P85 | SSRIB | 241.39 | 14.2 | 58.3 | -44.5 |
+| 21 | Wyffels | W5086RIB | - | 234 | 13.6 | 58.1 | -51.9 |