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-2025-9906.md

@@ -0,0 +1,37 @@
+# Corn yield trial — Dennis Lueder (West Union, IA, 2025)
+
+- **Publisher:** ProHarvest Seeds (cross-vendor plot report)
+- **Crop:** Corn
+- **Year:** 2025
+- **Location:** West Union, IA · Fayette County
+- **Cooperator:** Dennis Lueder
+- **Company rep:** Ryan Frederich
+- **Planted:** 5/6/2025
+- **Harvested:** 10/9/2025
+- **Row width:** 30
+- **Population:** 35,500 seeds/acre
+- **Tillage:** Conventional
+- **Irrigation:** Non-Irrigated
+- **Source PDF:** https://proharvestseeds.com/wp-content/uploads/2025/10/Corn-Lueder-West-Union-IA.pdf
+
+---
+
+## Results (head-to-head)
+
+| Rank | Brand | Hybrid/Variety | Trait | Yield/Ac | % H2O | Test Wt | +/- Ave |
+|---|---|---|---|---|---|---|---|
+| 1 | ProHarvest | 78P95 | PCE | 276.14 | 18.7 | 58.3 | 40.5 |
+| 2 | ProHarvest | 76P42 | PCE | 263.97 | 16.3 | 56.1 | 28.4 |
+| 3 | FS InVision | FS | 5947T RIB | 262.68 | 20.2 | 57.4 | 27.1 |
+| 4 | ProHarvest | 78P30 | VT2PRIB | 258.3 | 18.1 | 56.6 | 22.7 |
+| 5 | ProHarvest | 79P54 | SSPRO | 255.79 | 18.8 | 56.7 | 20.2 |
+| 6 | ProHarvest | 77P25 | VT4PRIB | 250.38 | 16.8 | 56 | 14.8 |
+| 7 | ProHarvest | 79P15 | V | 239.64 | 19.9 | 56.7 | 4 |
+| 8 | DEKALB | DKC105-33RIB | - | 232.29 | 16.8 | 53.6 | -3.3 |
+| 9 | ProHarvest | 75P85 | DGVT2PRIB | 230.58 | 16.9 | 55.2 | -5 |
+| 10 | ProHarvest | 79P87 | SSRIB | 226.03 | 16.6 | 57.8 | -9.6 |
+| 11 | ProHarvest | 71P16 | SSRIB | 214.61 | 15.9 | 58.4 | -21 |
+| 12 | ProHarvest | 71P16 | SSRIB | 211.66 | 15.3 | 57.3 | -23.9 |
+| 13 | ProHarvest | 71P16 | SSRIB | 207.47 | 16.4 | 56.9 | -28.1 |
+| 14 | ProHarvest | 71P16 | SSRIB | 204.36 | 15.9 | 56 | -31.2 |
+| 15 | ProHarvest | 71P16 | SSRIB | 200.21 | 15.1 | 58.3 | -35.4 |