Add comprehensive GI organoid toxicity screening example for NAMO

This commit adds a detailed example demonstrating how to model intestinal
organoid-derived epithelial monolayer systems for Phase IV drug metabolite
toxicity screening using the NAMO schema.

New files:
- tests/data/valid/Dataset-GI-toxicity-example.yaml: Complete working example
  of an OrganOnChip model with comprehensive multi-throughput functional assays
- tests/data/valid/GI-toxicity-example-README.md: Documentation explaining
  the example, usage instructions, and customization guidance

Key features of the example:

Model System (OrganOnChip):
- Human intestinal organoid-derived epithelial monolayers
- Dual-chamber microfluidic design with accessible apical/basal compartments
- Complete device specifications (TWO_CHANNEL architecture, porous polymer
  membrane, TEER sensors, collagen/fibronectin surface treatment)
- Mixed epithelial cell population (enterocytes, goblet cells, Paneth cells,
  enteroendocrine cells)

Multi-Throughput Functional Assays (17 total):
- High-throughput (96/384-well): Cell viability (MTT, ATP), cytotoxicity (LDH),
  calcium signaling (Fluo-4), ROS generation (DCF), ER stress markers
  (BiP/GRP78, CHOP), mitochondrial membrane potential (TMRE)
- Medium-throughput (24-well): TEER measurements, paracellular permeability
  (FITC-dextran, Lucifer Yellow)
- Low-throughput (microscopy): Junction integrity (ZO-1, occludin, E-cadherin),
  cell morphology/swelling, calcium dynamics (time-lapse), ER stress
  (confocal imaging, XBP1 splicing), redox state (GSH/GSSG), apoptosis
  (cleaved caspase-3)

Structured Concordance:
- Molecular similarity with gene expression analysis (VIL1, CDX2, MUC2, TJP1,
  OCLN) and statistical significance metrics
- Functional parity with detailed methodology for each of 17 assays including
  units, reference values, and protocols
- Cell type coverage with proportions and missing cell types
- Reproducibility with quality control metrics (confluence, TEER, viability)

Study Design:
- Context: Regulatory toxicology and Phase IV post-market drug safety
- Perturbations: Drug metabolites at 0.1-100 μM, acute (4-24h) and chronic
  (48-72h) exposure via apical chamber
- Endpoints: Comprehensive description organized by throughput level
- Validation: Comparison with known GI toxicants, Caco-2 cells, and FDA
  FAERS clinical adverse event data

This example demonstrates NAMO best practices for:
1. Organizing functional assays by throughput level
2. Providing quantitative methodology for each endpoint
3. Capturing device technical specifications
4. Linking model validation to clinical/regulatory context
5. Using structured concordance for comprehensive model characterization

The example validates successfully against the NAMO schema and serves as a
template for modeling similar organoid-based toxicity screening workflows.

Signed-off-by: Claude (Anthropic AI Assistant) <claude@anthropic.com>
Co-authored-by: Claude Code <noreply@anthropic.com>

Author: linkmluser