fix(connectivity): Use list instead of set for Net objects

[shanemmattner]

Summary

• Fix TypeError: unhashable type: 'Net' in ConnectivityAnalyzer._add_wire_to_net()
• Change existing_nets from set() to list() since Net objects are mutable and should not be hashable

Problem

When analyzing schematics with multiple wires connecting to the same pins, the connectivity analyzer crashed:

existing_nets = set()
for pin in pins:
    if pin in self._pin_to_net:
        existing_nets.add(self._pin_to_net[pin])  # TypeError: unhashable type: 'Net'

Net is a mutable dataclass (has merge(), add_pin() methods) and correctly does not define __hash__. But the code tried to add Net objects to a Python set.

Solution

Use a list with membership check instead:

existing_nets = []
for pin in pins:
    if pin in self._pin_to_net:
        net = self._pin_to_net[pin]
        if net not in existing_nets:
            existing_nets.append(net)

Test plan

• All 53 hierarchical connectivity tests pass
• Tested on real Motor Driver schematic (101 nets, 113 components)
• Existing ERC tests unaffected (were already failing before this change)

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