release: bump version to 0.1.2

[t-uda]

Summary

• update the package version from 0.1.2.dev1 to 0.1.2
• finalize the release notes heading as 0.1.2 dated 2026-03-25
• rebuild and verify the final release artifacts

Verification

• poetry run black --check src tests scripts
• poetry run flake8 src tests scripts
• poetry run mypy src
• MYPYPATH=src poetry run stubtest ellphi --allowlist stubtest-allowlist.txt
• poetry run python scripts/build_tangency_cpp.py
• poetry run pytest
• poetry run mkdocs build
• poetry build
• fresh-venv wheel install smoke test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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