Add NC to stdlib

[Hugo-Villagrana]

Note

Low Risk
Small, localized stdlib API change; main risk is minor compatibility/behavior differences for callers that relied on the old NotConnected() helper producing a plain Net.

Overview
Adds a new NotConnected entry to interfaces.zen as a builtin.net_type, making NC nets a first-class standard type.

Removes the previous def NotConnected(name: str = "NC") -> Net helper function, shifting usage from generating a plain Net to referencing the new net type.

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[Hugo-Villagrana]

Support for types: diodeinc/pcb#476