release: prepare v0.4.0

[bionicbutterfly13]

Summary

• bumps package version to 0.4.0
• publishes Track 026 changelog entry as the v0.4.0 release
• updates elume.__version__ to match pyproject.toml and makes the version test read pyproject directly
• updates README install pin to elume==0.4.0

Verification

• .venv/bin/pytest tests/unit/test_version.py tests/unit/envelope/test_snapshot.py tests/contract/test_provider_contract.py -q -> 65 passed
• .venv/bin/pytest -q -> 1194 passed
• .venv/bin/ruff check src tests reference_service/src -> clean
• uv build -> dist/elume-0.4.0.tar.gz and wheel built successfully