Prepare op_engine 0.4.0 release

[jc-macdonald]

Summary

• Prepare op-engine and flepimop2-op-engine 0.4.0.
• Require the matching 0.4.0 core from the provider, and raise its optional op_system integration floor to 0.6.0 (flepimop2-op-system>=0.6.0, op-system>=0.6.0).
• Point the development op_system pins at the op_system 0.6.0 release preparation (op_system #253, 89c34a6).
• Cut the accumulated changelog entries since 0.3.0 into the 0.4.0 section.

The provider's development flepimop2 source remains pinned to the reviewed tip of Flepimop2 PR #348. Published wheels continue to support the released flepimop2>=0.3.0 baseline.

Changelog edits

• Five merged changes had no entry: forcing-aware tau-leaping (Respect forcing boundaries in fixed and adaptive tau-leaping #174), producer forcing consumption (Consume declared producer forcing in stochastic execution #175), bounded thinning SSA (Add exact bounded thinning SSA for time-dependent rates #176, Expose bounded thinning SSA through the flepimop2 provider #177), and the renewal-birth fix (Support pinned renewal births and validate SSA population ensembles #179). I added them.
• I ordered the section by issue and removed stray blank lines.
• I added a Changed list for the raised op_system floor, and for pure stochastic runs now rejecting uncovered dynamics (Reject pure stochastic runs whose reactions do not cover the model #182).

Release order

1. Merge op_system #253 and publish op_system 0.6.0.
2. Move this PR's two op_system pins from 89c34a6 to #253's merge commit. I can do this.
3. Merge this PR and run the op_engine release workflow.

The op_engine release workflow runs just build-all. Its provider clean-room suite resolves the lowest published dependencies, so it needs op-system 0.6.0 on PyPI first.

Validation

• just ci: 592 core tests passed, 2 skipped. 379 provider tests passed. Ruff and strict mypy passed for both packages.
• just docs: strict documentation build passed.
• just release-check: release version 0.4.0 validates, including the exact op-engine==0.4.0 provider requirement.
• UV_PYTHON_VERSION=3.13 just build-check build-test-core: both wheels pass twine check --strict, and the clean core wheel suite passed (579 passed, 2 skipped).
• Not yet run: build-test-provider, which needs op_system 0.6.0 on PyPI.

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