fix(interop): reset dual mode to galgebra default in interop.Cl

[utiberious]

Problem

galgebra.interop.kingdon.Cl sets Ga.dual_mode('Iinv+') globally. If galgebra.interop.Cl is called afterward in the same session, it silently inherits 'Iinv+' even though its documented promise is galgebra defaults ('I+'). See #555.

from galgebra.interop.kingdon import Cl as KCl
from galgebra.interop import Cl

ga_k, *_ = KCl(3)   # sets Iinv+ globally
ga_g, *_ = Cl(3)    # silently still Iinv+ -- wrong

Fix

galgebra.interop.Cl now calls Ga.dual_mode('I+') before building the algebra, mirroring the symmetric reset that kingdon.Cl does for 'Iinv+'. Sequential calls to both variants in one session are now safe without manual save/restore.

Updated kingdon.Cl docstring: the .. warning:: save/restore block becomes a .. note:: explaining that interop.Cl resets the mode, with explicit warning that algebras built under Iinv+ will compute duals with I+ once a subsequent galgebra.interop.Cl call is made.

Added test_Cl_dual_mode_reset to prevent regression.

Long-term

Per-instance dual_mode_value on Ga is tracked on feat/issue-555-interop-dual-mode-per-instance (see #555). That removes the global mutation entirely.

See #555.

[utensil]

Thanks for the fix.

The one-line reset is correct and mirrors what kingdon.Cl already does for Iinv+. Two requests before merge:

1. No test for the actual bug. The existing tests never call KCl first, so they would pass without the fix. Please add:

from galgebra.interop.kingdon import Cl as KCl
from galgebra.interop import Cl
from galgebra.ga import Ga

KCl(3)
Cl(3)
assert Ga.dual_mode_value == 'I+'

2. PR body says "Fixes #555 (short-term)" which auto-closes 555 on merge. Change to "see #555" so the tracking issue stays open for the long-term per-instance fix.

One docstring nit: the note in kingdon.py says mixing is safe "as long as each call is followed by the code that uses that algebra before the next Cl call". Could be clearer that algebras built under Iinv+ will compute duals with I+ after a subsequent Cl() call.