Color MOSFET and op-amp by operating region

[esaruoho]

Summary

• MOSFETs display a colored channel indicator showing their operating region: gray for cutoff, blue for linear/triode, green for saturation
• Op-amps (both ideal OpAmpElm and real OpAmpRealElm) show a colored fill when the output is saturating: red tint for positive rail clipping, blue tint for negative rail clipping
• Feature is toggled via Other Options > Show Operating Region, persisted in localStorage, and off by default

Details

The MOSFET already tracks its operating region in the mode variable (0=cutoff, 1=linear, 2=saturation). This PR draws a wider colored line behind the channel segments to visually indicate the region. The colors are chosen to be distinct and meaningful: gray suggests no conduction, blue suggests resistive/linear behavior, and green suggests active/saturation operation.

For op-amps, the output voltage is compared against the supply rails. The ideal op-amp checks against maxOut/minOut, while the real op-amp checks against the actual supply voltages (volts[3]/volts[4]). When saturating, the triangle body is filled with a muted red or blue to indicate which rail the output is clipping to.

JFETs (JfetElm) inherit the MOSFET coloring automatically since they extend MosfetElm.

Files changed

• CircuitElm.java — added static boolean showOperatingRegion field and showOperatingRegion() accessor
• EditOptions.java — added "Show Operating Region" checkbox at index 14 (shifted existing options)
• UIManager.java — initialize showOperatingRegion from localStorage on startup
• MosfetElm.java — draw colored channel background based on mode (cutoff/linear/saturation)
• OpAmpElm.java — fill triangle when output is near supply rails
• OpAmpRealElm.java — fill triangle when output is near actual supply voltages

Test plan

• Place an N-MOSFET in a circuit, verify channel shows gray when Vgs < Vth
• Drive the MOSFET into linear region (Vds < Vgs-Vth), verify blue channel
• Drive the MOSFET into saturation (Vds > Vgs-Vth), verify green channel
• Verify P-MOSFET and JFET also show correct operating region colors
• Place an op-amp with output driven to positive rail, verify red fill
• Place an op-amp with output driven to negative rail, verify blue fill
• Verify op-amp shows no fill during normal linear operation
• Test with OpAmpRealElm (LM741/LM324) as well
• Toggle "Show Operating Region" off in Other Options, verify no coloring
• Verify option persists across page reloads via localStorage
• Verify existing rendering (voltage colors, power colors, dots) is unaffected

Addresses sharpie7#846.

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