Physical scale: detect the audited shape (no unit / [1,1,1]) and use the manifest pitch - #2220

Could a maintainer add the keep-open label? The last attempt (#2137) was closed by the PR time-limit
rule before anyone could act on the review; this one is the corrected version, and the label is what keeps it
alive long enough to be reviewed.

What was wrong with #2137, and what changed

@pmh47 was right on all three points: the audited stores publish an explicit [1,1,1] scale with no unit,
so a check that only fires when the scale transform is missing never warns on them; and the
metadata.min.json fallback the description promised was not in the diff at all. Both are fixed here.

1. Check the real condition. _axes_have_units() reads multiscales[0].axes[].unit and is independent of
   whether a transform is present; a level-0 scale of exactly [1, 1, 1] is also reported separately, since
   on a store whose name states a pitch that value is a placeholder rather than a measurement.
2. Implement the fallback. _pitch_from_manifest() resolves the store against metadata.min.json and
   reads properties.pixel_size_um. When the catalog records a pitch the warning names it as the value to
   use; when it does not (the six legacy fragments) the warning says so instead.
3. Tests pin the audited shape, not the missing-transform shape: units absent + explicit [1,1,1] must
   warn, units present must stay quiet, a channel axis without a unit must not count. A test that only covers
   the missing-transform case is exactly how the first version passed while doing nothing.

Calibration against the live catalog

PHerc0009B/volumes/20250521125136-8.640um-1.2m-116keV-masked.zarr  -> 8.64   (manifest pixel_size_um)
PHerc0009B/volumes/20250820154339-2.401um-0.3m-77keV-masked.zarr   -> 2.401
a store the manifest records no pitch for                         -> None

and the published metadata of that first store is the audited shape exactly:

axes:    [('z', None), ('y', None), ('x', None)]
level 0: [{'scale': [1.0, 1.0, 1.0], 'type': 'scale'}]

State of the diff: rebased onto main at 13a88a6, 10 files, +194/-2 in the first version and the
corrections above on top; every touched file compiles and the new tests are in lasagna/tests/. What I could
not do is run the repository's test suite here - the dependency chain is deep enough that I could not
reproduce it - so the CI run on this PR is the check I most want, and if anything fails I will fix it.

If the reviewer's preference is a different shape - for example failing hard rather than warning when the
catalog records a pitch and the store does not - say which and I will change it; the point of the patch is to
make the audited stores impossible to miss, not to prescribe a policy.