Add university-extension variety trials: Illinois VT + Iowa ICPT + Ohio OCPT (+123 trial docs)

Independent third-party performance data — land-grant programs that test every
entered brand side-by-side with replication + LSD stats. This is the legitimate
way to get Pioneer / DEKALB / Brevant / Channel performance the corpus can't
scrape directly (data_type=trial, results[] shape; falls through the trial
chunker).

- illinois_vt_trials (30 docs, 1,392 rows) — U of Illinois VT. Per-region XLSX
  (openpyxl), corn + soy + WHEAT, 2024+2025. Rich per-site agronomic metadata;
  corn-following-corn vs -soybean kept distinct.
- iowa_icpt_trials (24 docs, 674 rows) — Iowa State ICPT. ASP.NET GridView
  (viewstate postback for year/district), corn + soy by district x season.
- ohio_ocpt_trials (69 docs, 4,647 rows) — OSU/CFAES OCPT. Report PDF
  (pdfplumber; per-site column groups split by header Yield-token count +
  x-coord footnote bucketing), corn + soy per site, 2024+2025.

91 distinct seed brands across the three; majors confirmed present in the
independent rankings: DEKALB 395, Golden Harvest 249, Channel 241, NK 212,
Xitavo 135, LG 103, Pioneer 88, Asgrow 59. (A brand only appears where it
ENTERED a given program — e.g. Brevant not in Iowa, DEKALB/Channel not in
Illinois — true negatives, not parse gaps.)

- rag/chunk.py: gated `include_region` on _render_gh_plot_chunk; the 3 university
  sources route through it so the region/district is in the embedded chunk +
  labeled "variety trial (cross-vendor, independent third-party)". Existing plot
  sources (gh/lg/agrigold/proharvest) unchanged.
- requirements.txt: openpyxl (Illinois XLSX; scrape-time only).
- sources.json + README/CLAUDE/lessons: registered + attributed; lessons
  trial-data + Pioneer entries updated (Pioneer/DEKALB performance now available
  indirectly via these trials).

Validation: all 123 chunk via rag.chunk.chunks_from_trial (0 errors), 0
out-of-range yields, 0 dup keys. Public land-grant data; attribution recorded in
each tos_note. CI rebuilds the index from the committed corpus.

corpus/illinois_vt_trials/ilvt-corn-2025-r4.md

@@ -0,0 +1,35 @@
+# Corn yield trial — University of Illinois VT, Region 4, IL 2025
+
+- **Publisher:** University of Illinois Variety Testing (independent cross-vendor trial)
+- **Crop:** Corn
+- **State:** IL
+- **Year:** 2025
+- **Region:** Region 4
+- **Rotation:** corn following soybean
+- **Cooperator / host:** Emmerick Farms
+- **County:** Fayette
+- **Sites:** St Peter, Belleville
+- **Soil type:** Wynoose silt loam
+- **Tillage:** Vertical Till
+- **Planted:** 2025-06-02
+- **Harvested:** 2024-10-24
+- **Source XLSX:** https://vt.cropsci.illinois.edu/wp-content/uploads/2025/11/Region-4-Corn-Table-2025.xlsx
+- **Index page:** https://vt.cropsci.illinois.edu/corn/
+
+---
+
+## Results (ranked by regional yield)
+
+| Rank | Brand | Product | Traits | Yield | Moisture | Lodging | Maturity |
+|---|---|---|---|---|---|---|---|
+| 1 | Prairie | 8786 | - | 196.65 | 13.27 | 0 | 115 |
+| 2 | Burrus | 5C11 PCE | GT:C HT:B IST:M | 184.2 | 12.87 | 0 | 110 |
+| 3 | Burrus | 8K34 VT2P | GT:C HT:G IST:M | 177.59 | 14.15 | 0 | 116 |
+| 4 | Whisnand | WH403TRE | GT:C2L HT:G IST:L | 166.28 | 12.47 | 0 | 115 |
+| 5 | Prairie | 6854 | - | 157.49 | 12.25 | 0 | 110 |
+| 6 | Whisnand | WH304SS | GT:CRL HT:B IST:L | 157.45 | 12.78 | 0.33 | 112 |
+| 7 | Whisnand | WH303VT2P | GT:C2L HT:G IST:L | 157.26 | 12.97 | 0.17 | 115 |
+| 8 | Whisnand | WH402VT2P | GT:C2L HT:G IST:L | 155.69 | 12.85 | 0 | 114 |
+| 9 | Burrus | 7B14 PCE | GT:C HT:B IST:M | 152.92 | 13.02 | 0.33 | 114 |
+| 10 | Prairie | 7445 | - | 145.71 | 12.12 | 0 | 113 |
+| 11 | Prairie | 6436 | - | 137.95 | 12.45 | 0 | 111 |