Add Databricks validation scaffolding

[slusset]

Summary

• align the DLT pipeline outputs with the Databricks validation issue by producing writeback_log and dead_letters tables
• add Databricks notebooks for demo environment setup and validation summary reporting
• add a Databricks validation runbook to the docs site and link it from the README

Verification

• uv run ruff check src/ tests/
• uv run ruff format --check src/ tests/
• uv run pytest tests/unit -v --tb=short
• uv run --group docs mkdocs build --strict

Remaining blocker

• Issue Databricks DLT integration test with DABs deploy #6 still requires a real Databricks workspace run, HAPI validation, rerun/idempotency evidence, and screenshots or recording before it can be closed.