interop.Cl dual mode interaction: session contamination and per-instance fix

[utiberious]

Context

PR #550 added galgebra.interop.Cl (galgebra defaults) and galgebra.interop.kingdon.Cl (kingdon conventions). PR #544 showed that 'Iinv+' is the mathematically correct default for dual(), but changing the global default is a breaking change deferred past 0.6.0.

Remaining problem

Ga.dual_mode_value is a class-level attribute — session-wide global state. galgebra.interop.kingdon.Cl currently calls Ga.dual_mode('Iinv+') to set kingdon conventions, but this persists for the rest of the session. Any subsequent use of galgebra.interop.Cl (which promises galgebra defaults including 'I+' dual) silently inherits 'Iinv+':

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

ga_k, *_ = KCl(3)   # sets Iinv+ globally
ga_g, *_ = Cl(3)    # does NOT reset — silently still Iinv+!
ga_g.mv('a', 'vector').dual()  # wrong convention for galgebra.interop.Cl

Short-term fix (0.6.0)

• Make galgebra.interop.Cl explicitly call Ga.dual_mode('I+') to reset to galgebra defaults, mirroring what kingdon.Cl does for Iinv+.
• Add a docstring note on both Cl variants documenting the session-wide nature of the dual mode and the interaction between the two.
• Update the .. warning:: in kingdon.Cl accordingly.

This preserves the existing Ga.dual_mode() global API and is safe for 0.6.0.

Long-term fix (post-0.6.0)

Make dual_mode_value a per-instance attribute on Ga. Since mv.py already reads self.Ga.dual_mode_value (instance-lookup syntax), the structural change is minimal:

1. Ga.__init__: self.dual_mode_value = kwargs.pop('dual_mode', Ga.dual_mode_value) — snapshot the class default (or accept an explicit override) at creation time.
2. Ga.build(): forward dual_mode= kwarg.
3. galgebra.interop.Cl: pass dual_mode='I+' to Ga.build() instead of mutating global state.
4. galgebra.interop.kingdon.Cl: pass dual_mode='Iinv+' to Ga.build() instead of mutating global state.
5. Remove the .. warning:: save/restore block from kingdon.Cl — no longer needed.

Ga.dual_mode() class setter keeps working for backward compat. mv.py needs no changes.

References

• fix(ga): change default dual mode to Iinv+ for correct B*dual(B)=I #544 — proof-of-concept for 'Iinv+' as the correct global default
• feat(ga): add Cl(p,q,r) kingdon-like interface for Clifford algebras #550 — merged Cl(p,q,r) interface with namespace-based conventions

[utiberious]

Long-term per-instance refactor confirmed as post-0.6.0. Branch feat/issue-555-interop-dual-mode-per-instance is parked. Short-term fix (PR #556) targets 0.6.0.