Fix SCR oscillation: use latching trigger/hold logic like TriacElm

The previous combined formula (ig/triggerI + ia/holdingI > 1) re-evaluated
the on/off state every timestep without hysteresis.  With a series resistor,
the anode current would hover near the threshold: the SCR would turn on,
current would rise, then the voltage drop across the series resistor would
reduce anode current below the threshold, turning it off again -- causing
oscillation between two states.

Replace with separate trigger and hold conditions:
- Gate current exceeding triggerI latches the SCR on (state = true)
- Anode current dropping below holdingI turns it off (state = false)

This matches the latching behavior of a real SCR and the pattern already
used by TriacElm.  An SCR with a series resistor should latch on and remain
on once triggered, which is what LTspice shows for the same circuit.

Also reset state to false in reset() for proper initialization.

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

Author: esaruoho

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

@@ -76,6 +76,7 @@ void reset() {
 	volts[anode] = volts[cnode] = volts[gnode] = 0;
 	diode.reset();
 	lastvag = lastvac = curcount_a = curcount_c = curcount_g = 0;
+	state = false;
     }
     int getDumpType() { return 177; }
 
@@ -222,12 +223,17 @@ void stamp() {
     // 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.
+    // preventing convergence (issue #851).
+    //
+    // Use separate trigger/hold conditions with latching, matching the pattern
+    // used by TriacElm.  Once the gate current exceeds triggerI the SCR latches
+    // on, and stays on until the anode current drops below holdingI.  The gate
+    // only triggers; it does not need to remain driven to keep the SCR on.
     void startIteration() {
-	double icmult = 1/triggerI;
-	double iamult = 1/holdingI - icmult;
-	state = (-icmult*ic + ia*iamult > 1);
+	if (ia < holdingI)
+	    state = false;
+	if (ig > triggerI)
+	    state = true;
 	aresistance = state ? .0105 : 10e5;
     }