feat(PhyslibAlpha/EM): Kirchhoff Current Law

[jstoobysmith]

Kirchhoff's current law for the functional and distributional cases. Made with Claude Code Opus 5.5.

[github-actions]

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[morrison-daniel]

Looks good

[zhikaip]

I have reservations about calling these results Kirchhoff's current law. To me KCL is a statement about branch currents at the nodes of a lumped-element circuit, while the box result is just integral charge conservation. The wire-junction result is closer, but covers a single node with steady currents, so it is a special case rather than KCL itself.

For PhyslibAlpha I'm fine with the implementation as a toy example, but I'd recommend renaming so the names don't suggest more than is proved, e.g. boxOutwardCurrent_eq_zero_of_steady and IsWireJunction.sum_currents_eq_zero, with the file names and module docstrings adjusted to match. The docstrings could mention KCL as motivation.

[jstoobysmith]

@zhikaip will make this change before merging

[jstoobysmith]

@zhikaip Made the change

[zhikaip]

Lgtm