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-2025-9874.md

@@ -0,0 +1,40 @@
+# Corn yield trial — Jeff Brockman (Ransom, IL, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** Ransom, IL · Lasalle County
+- **Cooperator:** Jeff Brockman
+- **Company rep:** John Walsh
+- **Planted:** 4/27/2025
+- **Harvested:** 10/14/2025
+- **Previous crop:** Soybeans
+- **Row width:** 30
+- **Tillage:** Strip/Zone
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Brockman-Ransom-IL.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | Agrigold | A645-71 | STXRIB | 255.79 | 17.7 | None | 23.7 |
+| 2 | Wyffels | W7759RIB | - | 248.87 | 16.2 | None | 16.8 |
+| 3 | DEKALB | DKC110-10RIB | - | 247.98 | 16.5 | None | 15.9 |
+| 4 | ProHarvest | 74P10 | PCE | 246.57 | 15 | None | 14.5 |
+| 5 | DEKALB | DKC112-35RIB | - | 245.39 | 16.4 | None | 13.3 |
+| 6 | DEKALB | DKC114-42RIB | - | 239.02 | 17.6 | None | 6.9 |
+| 7 | Agrigold | A642-18 | SSP | 238.18 | 16.9 | None | 6.1 |
+| 8 | ProHarvest | 83P48 | SSPRO | 235.6 | 16.8 | None | 3.5 |
+| 9 | DEKALB | DKC107-11 | - | 233.17 | 15.6 | None | 1.1 |
+| 10 | Wyffels | W7499RIB | - | 229.58 | 16.9 | None | -2.5 |
+| 11 | ProHarvest | 79P54 | SSPRO | 229.16 | 16 | None | -2.9 |
+| 12 | Wyffels | W6479RIB | - | 227.07 | 15.7 | None | -5 |
+| 13 | DEKALB | DKC109-71RIB | - | 225.49 | 15.2 | None | -6.6 |
+| 14 | ProHarvest | 79P15 | V | 222.83 | 16.2 | None | -9.3 |
+| 15 | ProHarvest | 78P95 | PCE | 222.3 | 16.4 | None | -9.8 |
+| 16 | ProHarvest | 81P20 | PCE | 212.01 | 15.9 | None | -20.1 |
+| 17 | Wyffels | W4439RIB | - | 211.34 | 15 | None | -20.7 |
+| 18 | ProHarvest | 82P96 | PCE | 207.12 | 16.7 | None | -25 |