chore: release 0.3.0

[maish]

First PyPI release. Bumps pyproject.toml to 0.3.0 and promotes [Unreleased] → ## [0.3.0] — 2026-05-06 in the CHANGELOG.

The [0.2.0] section in CHANGELOG describes work prepared but never tagged/published; it stays as a historical record. v0.3.0 is the first version that will exist on PyPI.

Test plan

• All 7 required CI checks green (lint, build, integration, test py3.9-3.12)
• After merge: tag v0.3.0, confirm release.yml publishes to PyPI via Trusted Publishing
• pip install table-stitcher==0.3.0 works from a clean venv