⬆️ dep-bump: cap unxt below 2.0

[nstarman]

galax declares unxt>=1.11.2 with no upper bound, but has never been
resolved or tested against unxt 2.x — the lock has only ever held a 1.x.

Nothing currently declares that boundary. uv does consider unxt 2.0.0
during resolution (it is compatible on requires-python) and backtracks to
1.11.x for transitive reasons. That is incidental rather than a guarantee, and
it disappears as soon as the surrounding constraints shift — a coordinax bump,
a new dependency, an --upgrade.

This makes the tested boundary explicit. The resolved version is unchanged
— 1.11.2, as before — so the diff is metadata only:

- { name = "unxt", specifier = ">=1.11.2" },
+ { name = "unxt", specifier = ">=1.11.2,<2" },

What it guards against

unxt is making unit-system repr/str round-trippable
(GalacticDynamics/unxt#855):

	before	after
repr	unitsystem(kpc, …) or LengthUnitSystem(length=Unit("km"))	unitsystem('kpc', 'Myr', 'solMass', 'rad')
str	LTMAUnitSystem(length, time, mass, angle)	unitsystem(kpc, Myr, solMass, rad)

That rewrites the LTMAUnitSystem( length=Unit("kpc"), ...) form which 32
doctest lines here pin — 18 in _interop/galax_interop_gala/potential.py, 12
in _interop/galax_interop_galpy/potential.py, and one each in
potential/_src/builtin/burkert.py and potential/_src/params/field.py.

Those updates belong with the port to unxt 2.x rather than ahead of it, since
2.0 already carries its own breaking changes (no-arg unitsystem() is now
dimensionless, unit-system identity is order-independent, Python 3.11 dropped).

Lift this cap in the PR that does that port.

Verification

uv lock produces no resolved-version change. tests/smoke 8 passed;
tests/unit 1032 passed, 170 skipped, 14 xfailed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.86%. Comparing base (e6ef7d5) to head (d0fea38).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #800   +/-   ##
=======================================
  Coverage   95.86%   95.86%           
=======================================
  Files         160      160           
  Lines        6046     6046           
=======================================
  Hits         5796     5796           
  Misses        250      250           

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