Add MOSFET channel-length modulation and scope trail persistence

[esaruoho]

Summary

• MOSFET channel-length modulation (fixes MOSFET output resistance #155): Adds a lambda parameter to MosfetElm/JfetElm for finite output resistance in saturation. In saturation region: Ids = 0.5beta(Vgs-Vt)^2*(1+lambdaVds), Gds = 0.5beta*(Vgs-Vt)^2*lambda. Default is 0 (ideal square-law, backward compatible). Shown in the info panel when non-zero.
• Scope XY trail persistence (fixes [FEATURE] Option to configure the fade-out time of the oscilloscope x-y trail #157): Adds a "Trail Persistence" slider (0-50) in the scope properties dialog under XY Plots. Values 0-49 control fade speed (lower = faster fade); 50 = infinite persistence (no fade). Default is 2 (matches previous hardcoded behavior).

Changes

• MosfetElm.java: Added lambda field, channel-length modulation math in saturation, serialization, edit dialog entry at n=2 (shifted subsequent items)
• JfetElm.java: Extended getEditInfo threshold from n < 2 to n < 3 to expose lambda
• Scope.java: Added trailLen field, changed fade condition to use configurable threshold
• ScopePropertiesDialog.java: Added trail persistence scrollbar and label after XY plot checkboxes

Test plan

• Place an n-MOSFET, set lambda > 0 (e.g. 0.02), verify finite output resistance in saturation (Ids increases with Vds)
• Place a JFET, verify lambda parameter appears in edit dialog
• Open scope XY plot, adjust Trail Persistence slider and verify fade behavior changes
• Verify lambda=0 (default) gives identical behavior to before
• Verify existing circuits load without issues (backward-compatible dump format)

[esaruoho]

Superseded by #240 which targets v3-dev instead of master. Closing this PR.