Initial setup for LabelingIR, defining the foundational Intermediate Representation (IR) structs for the North Shore labeling stack.

This commit introduces:
- License file (MIT) and basic project structure files (`mix.exs`, `.formatter.exs`, `hex_metadata.config`).
- README with installation, usage examples, and structure overview.
- Core struct definitions for: Sample, Schema, Field, Assignment, Label, Dataset, EvalRun, Artifact, and ArtifactRef.
- Added `Jason.Encoder` derivation to all structs for JSON serialization compatibility.
- Added optional `namespace` and optional `lineage_ref` (map) fields across all core entities for multi-tenancy and provenance tracking.
- Updated gitignore to exclude Dialyzer PLT files.

See README for detailed struct fields and usage.