Bump version to 0.3.0

[jcschaff]

Version bump to 0.3.0 ahead of the release.

Since 0.2.0 the project gained (PRs #41–#48):

• MathDescription data model exposed in the VCML reader/writer (per-application generated math)
• Advection velocity on species mappings; add_model_parameter role defaults to "user defined" (matches VCell Java)
• Lightweight data layer: slim core install + optional-dependency extras (pip install pyvcell[all] for the full stack), lazy (PEP 562) imports
• Lenient VCML reader: best-effort physiology parse (real-biomodel corpus load 77% → 100%)
• Volume-variable shape robustness in the zarr writer; libvcell>=0.0.15.3
• Maintenance: dropped Python 3.9/3.10/3.11 from CI (requires-python >=3.12), removed poetry

The release workflow sets the published version from the release tag, but this keeps the in-repo pyproject.toml / uv.lock in sync.

🤖 Generated with Claude Code