Single-source the version, and stop the local test run lying

[benjipeng]

Three fixes to the build and test surface, all found by actually running the commands the README tells a developer to run.

The version is read from one place

pyproject.toml declared a literal alongside the one in __init__.py. Nothing failed loudly when they disagreed — a wheel would just misreport itself. It is now dynamic = ["version"] with [tool.hatch.version] pointing at __init__.py.

Proved genuinely dynamic rather than coincidental: setting __version__ to 9.9.9 builds mlx_cv-9.9.9-py3-none-any.whl. Restored immediately; git diff on __init__.py is empty.

pytest from the repository root no longer detonates

There were no testpaths, so collection walked into references/, where mlx-vlm's smoke test calls sys.exit(1) at import time and takes the whole run down with an INTERNALERROR.

CI never saw this because references/ is untracked — so the breakage was invisible to the one place that runs the suite automatically, and unavoidable on every developer machine that has the upstream checkouts.

Two heavy gates reported environment gaps as parity failures

Both checked that the external assets were present but not that they could be read:

• the SAM 3.1 contract needs PyTorch
• the DA3 video case needs OpenCV

With the weights on disk and neither package installed, they failed rather than skipped — red that says "parity broke" when nothing had been compared at all. That is the worst kind of red, because it trains people to ignore it.

They skip with the reason now. DA3's required mode still fails, so a gate that is supposed to run cannot vanish quietly. This follows the pattern the DA3 suite already used elsewhere (_or_skip helpers, pytest.importorskip("cv2")) — the failing paths had simply missed it.

Result

	before	after
bare pytest	INTERNALERROR	454 passed
with testpaths only	2 failed, 454 passed	—
final		454 passed, 17 skipped, 0 failed

Both gates verified to emit a reason:

SKIPPED tests/test_sam31_reference_contract.py:164: official SAM 3.1 checkpoint inspection requires PyTorch
SKIPPED tests/test_da3_upstream_parity.py:316: DA3 video frame loading requires OpenCV/cv2