Fix SCR convergence failure with series resistor (sharpie7#851)

Move the SCR on/off state evaluation from doStep() to startIteration()
so it is evaluated once per timestep rather than on every Newton-Raphson
sub-iteration.  The previous code switched the anode resistance between
0.0105 ohms (on) and 100K ohms (off) inside doStep(), causing the
resistance to flip-flop on every iteration when the SCR was near its
trigger threshold with an external series resistor, preventing
convergence.

This matches the pattern used by TriacElm and DiacElm, which both
evaluate their state in startIteration().

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: esaruoho

src/com/lushprojects/circuitjs1/client/SCRElm.java

@@ -97,6 +97,7 @@ void undumpXml(XMLDeserializer xml) {
     double ia, ic, ig, curcount_a, curcount_c, curcount_g;
     double lastvac, lastvag;
     double gresistance, triggerI, holdingI;
+    boolean state;
 
     final int hs = 8;
     Polygon poly;
@@ -217,6 +218,19 @@ void stamp() {
 	diode.stamp(nodes[inode], nodes[cnode]);
     }
 
+    // Evaluate the on/off state once per timestep, not during Newton-Raphson
+    // iterations.  Previously the state switching was inside doStep(), causing
+    // the anode resistance to flip-flop between on (0.0105) and off (10e5) on
+    // every sub-iteration when an external series resistance was present,
+    // preventing convergence (issue #851).  This matches the pattern used by
+    // TriacElm and DiacElm.
+    void startIteration() {
+	double icmult = 1/triggerI;
+	double iamult = 1/holdingI - icmult;
+	state = (-icmult*ic + ia*iamult > 1);
+	aresistance = state ? .0105 : 10e5;
+    }
+
     void doStep() {
 	double vac = volts[anode]-volts[cnode]; // typically negative
 	double vag = volts[anode]-volts[gnode]; // typically positive
@@ -226,11 +240,6 @@ void doStep() {
 	lastvac = vac;
 	lastvag = vag;
 	diode.doStep(volts[inode]-volts[cnode]);
-	double icmult = 1/triggerI;
-	double iamult = 1/holdingI - icmult;
-	//System.out.println(icmult + " " + iamult);
-	aresistance = (-icmult*ic + ia*iamult > 1) ? .0105 : 10e5;
-	//System.out.println(vac + " " + vag + " " + sim.converged + " " + ic + " " + ia + " " + aresistance + " " + volts[inode] + " " + volts[gnode] + " " + volts[anode]);
 	sim.stampResistor(nodes[anode], nodes[inode], aresistance);
     }
     void getInfo(String arr[]) {