Configure python-semantic-release in pyproject.toml

[ldgaetano]

Add [tool.semantic_release] configuration to pyproject.toml so that conventional commits drive automatic semver version bumps.

Changes:

• Add [tool.semantic_release] with version_toml, tag_format, branch
• Add [tool.semantic_release.branches.main] to restrict releases to main
• Add [tool.semantic_release.changelog] with changelog_file and mode
• Add python-semantic-release to pixi dev dependencies
• Add pixi task: build → python -m build

Part of the CI/CD setup plan.