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-2024-urbana-cfc.md

@@ -0,0 +1,31 @@
+# Corn yield trial — University of Illinois VT, Urbana CFC, IL 2024
+
+- **Publisher:** University of Illinois Variety Testing (independent cross-vendor trial)
+- **Crop:** Corn
+- **State:** IL
+- **Year:** 2024
+- **Region:** Urbana CFC
+- **Rotation:** corn following corn
+- **Previous crop:** corn
+- **Cooperator / host:** Darin Joos
+- **County:** Chamapign
+- **Sites:** Urbana
+- **Soil type:** Drummer silty clay loam
+- **Tillage:** None
+- **Planted:** 2024-05-20
+- **Harvested:** 2024-10-18
+- **Source XLSX:** https://vt.cropsci.illinois.edu/wp-content/uploads/2024/11/Urbana-CFC-Table-2024.xlsx
+- **Index page:** https://vt.cropsci.illinois.edu/corn/
+
+---
+
+## Results (ranked by regional yield)
+
+| Rank | Brand | Product | Traits | Yield | Moisture | Lodging | Maturity |
+|---|---|---|---|---|---|---|---|
+| 1 | Prairie | 6755 | - | 190.876 | 16.64 | 0.33 | 110 |
+| 2 | Prairie | 6854 | - | 179.6544 | 14.33 | 1 | 110 |
+| 3 | Prairie | 7184 | - | 176.0747 | 16.85 | 1 | 112 |
+| 4 | Prairie | 8864 | - | 170.8097 | 18.23 | 1 | 114 |
+| 5 | Prairie | 8683 | - | 164.3276 | 20.2 | 0.67 | 115 |
+| 6 | Prairie | 9703 | - | 158.9303 | 18.57 | 1 | 116 |