test: cover flow2rgb validation guards and Flow2Rgb max_norm caching

[wkentaro]

flow2rgb and Flow2Rgb only had happy-path coverage; the three input-validation
guards and the class's max_norm caching were untested.

Summary

• Cover the ndim, channel-count, and dtype ValueError guards in flow2rgb.
• Cover the return_max=True branch (reports the computed max norm, output stays
  consistent when that norm is reused).
• Cover Flow2Rgb caching the first frame's max_norm and not recomputing it on
  later frames, plus an explicitly-constructed max_norm being preserved.

Test plan

• uv run pytest tests/unit/_flow_test.py (9 passed)
• full suite uv run --extra all pytest -n=auto tests (257 passed)
• uv run ruff check, ruff format --check, ty check on the changed file