chore(project): add pyproject.toml and project configuration

Configure Python project with pydantic, structlog, typer, rich dependencies.
Set up ruff, mypy, pytest tooling with strict type checking.

CLAUDE.md

@@ -0,0 +1,116 @@
+# CLAUDE.md
+
+Guidelines for working on the Veritext project.
+
+## Project Overview
+
+Veritext is a semantic text validation framework for Python. It validates text outputs
+against quality criteria using metrics like BLEU, ROUGE, and semantic similarity.
+
+## Directory Structure
+
+```
+veritext/
+├── src/veritext/          # Package source
+│   ├── core/              # Shared types, tokenisation, config
+│   ├── metrics/           # BLEU, ROUGE, lexical, readability
+│   ├── semantic/          # Optional embedding-based similarity
+│   ├── validators/        # Composable validation checks
+│   ├── benchmark/         # Quality tracking & regression detection
+│   ├── pytest_plugin/     # Native pytest integration
+│   └── cli/               # Command-line interface
+├── tests/                 # Test suite (mirrors src structure)
+├── docs/                  # Project documentation
+└── examples/              # Usage examples
+```
+
+## Code Style
+
+### Python Conventions
+
+- **Python 3.11+** with modern type hints
+- **UK English** in all text (colour, behaviour, summarisation, tokenisation)
+- **snake_case** for variables, functions, modules
+- **PascalCase** for classes
+- Absolute imports from package root: `from veritext.core.types import ...`
+
+### Quality Gates
+
+All must pass with zero issues before any commit:
+
+```bash
+uv run ruff check .              # Linting
+uv run ruff format --check .     # Formatting
+uv run mypy src/                 # Type checking
+uv run pytest                    # Tests
+```
+
+### Documentation
+
+- Docstrings for all public APIs (Google style)
+- Type hints on all function signatures
+- Keep docstrings concise; let types speak where possible
+
+## Architecture
+
+### Layer Dependencies
+
+```
+CLI / pytest_plugin  (presentation)
+        ↓
+validators / benchmark  (decision logic)
+        ↓
+metrics  (pure computation)
+        ↓
+core  (shared types, tokenisation)
+```
+
+Each layer depends only on layers below it.
+
+### Metrics vs Validators
+
+| Concept | Responsibility | Output |
+|---------|----------------|--------|
+| **Metric** | Compute a score | Typed result (e.g., `BleuResult`) |
+| **Validator** | Make pass/fail decision | `ValidationResult` with diagnostics |
+
+### Edge Case Handling
+
+- Empty text: Metrics return zero scores; validators fail
+- Empty reference: Comparison metrics raise `ValueError`
+- Whitespace-only: Treated as empty after tokenisation
+- Unicode: NFC normalisation by default
+
+## Git Workflow
+
+### Commits
+
+- Format: `type(scope): description`
+- Types: feat, fix, chore, refactor, docs, test
+- Atomic: ≤3 new files, ≤150 LOC per commit
+- Update changelog.md before completing a task
+
+### Branches
+
+- `feat/kebab-case` — new features
+- `fix/kebab-case` — bug fixes
+- `chore/` — maintenance
+- `refactor/` — code restructure
+- `docs/` — documentation only
+
+## Testing
+
+- Test files mirror source structure: `tests/test_core/test_types.py`
+- Use pytest fixtures for common setup
+- Target ≥80% coverage
+- Include edge cases: empty input, Unicode, boundary values
+
+## Pre-Completion Checklist
+
+Before marking ANY task complete:
+
+- [ ] All linting/formatting/type checks pass
+- [ ] Tests pass with adequate coverage
+- [ ] changelog.md updated if user-facing changes
+- [ ] Filenames are lowercase (except CLAUDE.md)
+- [ ] Commit follows `type(scope): description` format