Add example circuits from community contributions

[esaruoho]

Summary

• Add MOSFET super-diode example circuit: N-channel MOSFET configured as an ideal diode replacement with near-zero forward voltage drop (from sharpie7/circuitjs1#415)
• Add switching regulator example circuit: Buck regulator with bridge rectifier input, feedback loop using op-amp comparator (from sharpie7/circuitjs1#404)
• Add RTL logic gates example circuit: Complete set of RTL gate implementations (NOT, AND, OR, NAND, NOR, XOR, XNOR) built from discrete transistors and resistors (from sharpie7/circuitjs1#909)
• Add star-delta transformation example circuit: Demonstrates star (wye) to delta (triangle) resistor network equivalence (from sharpie7/circuitjs1#719)

Each circuit is placed in the appropriate section of setuplist.txt:

• MOSFET super-diode → MOSFETs section
• Switching regulator → Power Converters section
• RTL gates → RTL subsection under Logic Families
• Star-delta → Other Passive Circuits section (next to Wheatstone Bridge)

All circuit data was sourced directly from the original issue submissions.

Test plan

• Load each circuit from the Circuits menu and verify it simulates correctly
• Verify MOSFET super-diode shows near-zero forward voltage drop on scope
• Verify switching regulator output stabilizes at ~3.3V
• Verify RTL gates produce correct logic outputs for all gate types
• Verify star-delta transformation shows equivalent impedance between configurations
• Confirm no existing menu entries are broken

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