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-2025-9826.md

@@ -0,0 +1,49 @@
+# Corn yield trial — Brian Egel (Nichols, IA, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** Nichols, IA · Muscatine County
+- **Cooperator:** Brian Egel
+- **Company rep:** Jake Ekstrand
+- **Planted:** 4/27/2025
+- **Harvested:** 10/2/2025
+- **Previous crop:** Soybeans
+- **Row width:** 30
+- **Population:** 34,000 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Egel-Nichols-IA-min.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 78P95 | PCE | 230.09 | 11.8 | 59 | 31.5 |
+| 2 | ProHarvest | 81P19 | VT4PRIB | 230.02 | 12 | 59 | 31.4 |
+| 3 | ProHarvest | 84P78 | TRERIB | 224.91 | 12.4 | 60 | 26.3 |
+| 4 | Cornelius | 7399 | PCE | 221.85 | 12.2 | 58 | 23.3 |
+| 5 | ProHarvest | 82P96 | PCE | 220.06 | 12.2 | 58 | 21.5 |
+| 6 | ProHarvest | 83P66 | VT2PRIB | 219.37 | 11.4 | 58 | 20.8 |
+| 7 | ProHarvest | 79P15 | V | 214.35 | 12.1 | 59 | 15.8 |
+| 8 | Cornelius | C7235 | PCE | 210.01 | 12.1 | 58 | 11.4 |
+| 9 | Cornelius | C7448 | PCE | 205.64 | 13.2 | 58 | 7.1 |
+| 10 | Cornelius | C7202 | SSP | 204.72 | 11 | 60 | 6.1 |
+| 11 | Brevant | B13C49 | PCE | 203.16 | 11 | 58 | 4.6 |
+| 12 | ProHarvest | 83P48 | SSPRO | 202.39 | 12.7 | 59 | 3.8 |
+| 13 | Cornelius | C7120 | - | 201.9 | 12.6 | 59 | 3.3 |
+| 14 | ProHarvest | 79P54 | SSPRO | 201.45 | 11.6 | 59 | 2.9 |
+| 15 | ProHarvest | 81P20 | PCE | 193.04 | 11.9 | 57 | -5.5 |
+| 16 | Cornelius | C7216 | VT4P | 192.35 | 15.1 | 59 | -6.2 |
+| 17 | Cornelius | C7021 | DP | 191.97 | 12 | 59 | -6.6 |
+| 18 | Cornelius | 7342 | SSP | 190.62 | 12.6 | 58 | -8 |
+| 19 | ProHarvest | 81P45 | TRERIB | 189.83 | 11.1 | 58 | -8.7 |
+| 20 | ProHarvest | 77P25 | VT4PRIB | 188.3 | 10.7 | 57 | -10.3 |
+| 21 | Cornelius | C7366 | DGDP | 187.69 | 12.1 | 58 | -10.9 |
+| 22 | ProHarvest | 85P58 | VT2PRIB | 187.53 | 12.8 | 60 | -11 |
+| 23 | Brevant | B12H48 | PCE | 169.56 | 11.3 | 57 | -29 |
+| 24 | Cornelius | C6847 | TRE | 167.93 | 12 | 58 | -30.6 |
+| 25 | Cornelius | C7373 | SSP | 164.54 | 12 | 58 | -34 |
+| 26 | ProHarvest | 86P55 | TRERIB | 149.62 | 14.2 | 56 | -49 |