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-2024-9521.md

@@ -0,0 +1,43 @@
+# Corn yield trial — Isu Farm-EXP (Lexington, IL, 2024)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2024
+- **Location:** Lexington, IL · McLean County
+- **Cooperator:** Isu Farm-EXP
+- **Company rep:** Eric Yoder
+- **Planted:** 5/23/2024
+- **Harvested:** 10/21/2024
+- **Previous crop:** Soybeans
+- **Row width:** 30
+- **Population:** 35,000 seeds/acre
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2024/11/Corn-ISU-Farm-Experimental-Normal-IL-61702.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 82P96 | PCE | 298.55 | 16 | 57.5 | 18.2 |
+| 2 | Pioneer | P1742 | Q | 295.44 | 18.1 | 56.7 | 15.1 |
+| 3 | ProHarvest | 82P82 | VT2PRIB | 293.84 | 14.3 | 59.1 | 13.5 |
+| 4 | ProHarvest | X24797 | - | 293.7 | 17.3 | 58.8 | 13.4 |
+| 5 | ProHarvest | 8357 | DGVT2PRIB | 292.48 | 16.1 | 59.4 | 12.2 |
+| 6 | ProHarvest | X24728 | - | 290.65 | 15.6 | 57.6 | 10.3 |
+| 7 | ProHarvest | 81P20 | PCE | 290.4 | 16.4 | 55.6 | 10.1 |
+| 8 | ProHarvest | X24794 | - | 289.62 | 16.1 | 60.4 | 9.3 |
+| 9 | ProHarvest | X24742 | - | 284.5 | 15.1 | 60.6 | 4.2 |
+| 10 | ProHarvest | 81P20 | PCE | 284.02 | 16.7 | 54.6 | 3.7 |
+| 11 | ProHarvest | 81P20 | PCE | 282.15 | 15.7 | 57.1 | 1.8 |
+| 12 | Pioneer | P10477V | - | 279.25 | 15.5 | 56.4 | -1.1 |
+| 13 | ProHarvest | 81P20 | PCE | 277.74 | 16.3 | 56.7 | -2.6 |
+| 14 | ProHarvest | X24755 | - | 276.51 | 18.3 | 56.5 | -3.8 |
+| 15 | ProHarvest | X24680 | - | 276.12 | 14.9 | 57.3 | -4.2 |
+| 16 | Pioneer | P10625V | - | 273.4 | 14.7 | 58.5 | -6.9 |
+| 17 | Pioneer | P08527V | - | 268.91 | 15.2 | 58.4 | -11.4 |
+| 18 | ProHarvest | X24640 | - | 267.69 | 16 | 57.8 | -12.6 |
+| 19 | ProHarvest | X24724 | - | 266.58 | 17.6 | 56.2 | -13.7 |
+| 20 | ProHarvest | X24700 | - | 259.68 | 17.4 | 57.7 | -20.6 |
+| 21 | ProHarvest | X24790 | - | 245.11 | 18.5 | 56.5 | -35.2 |