Test the determinism contract of hflow.testing.joint_sine_parameters #34

Current behavior

joint_sine_parameters (src/hflow/testing.py:220) is a public function in hflow.testing with a determinism contract in its docstring ("derived only from the spec"), and the synthetic-episode generator depends on it (src/hflow/testing.py:545). It has zero direct test coverage: nothing pins the contract that the same SyntheticEpisodeSpec always produces the same parameters, which is what makes synthetic test episodes reproducible across runs and machines.

Proposed change

Add a small behavioral test in tests/ covering the function's contract:

• Determinism: two calls with specs sharing the same seed and joint_count return identical parameters.
• Frequency span: the first joint gets JOINT_SINE_MIN_FREQUENCY_HZ, the last joint gets JOINT_SINE_MAX_FREQUENCY_HZ, and frequencies are evenly spaced between them.
• Edge case: joint_count=1 returns a single pair at the minimum frequency (the max(spec.joint_count - 1, 1) guard).
• Phase range: every phase is in [0, 2*pi).

Definition of done

• The test asserts the observable contract above, not implementation details (per CONTRIBUTING.md: no tests that only assert how a helper is built internally).
• Place it alongside the closest existing behavioral tests for hflow.testing.

Validation

uv run pytest -q
uv run ruff check --fix && uv run ruff format
uv run ty check