Normalize run weights before stochastic event distribution

distributeEvents drew randomNumber from U[0,1] and compared it to a
running sum of the raw file weights. The weights are relative and not
required to sum to 1, so for a file like "11 1 / 12 7 / 13 2" (cumulative
bins 1/8/10) every draw is <= 1 and the first run always wins — runs 12
and 13 get zero events, with no warning. The code only worked when the
weights happened to sum to 1.

Compute the total weight once and scale each draw by it (and guard a
non-positive total). Scaling the draw rather than mutating runWeights
keeps the logged weight table showing the user's original values.

Verified: a 1/7/2 weights file over 1000 events now distributes 97/697/206
(~10/70/20%); before, all 1000 went to the first run (Geant4 11.4.1 dev
container).

Fixes #103

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: zhaozhiwen

gemc/eventDispenser/eventDispenser.cc

@@ -104,10 +104,19 @@ void EventDispenser::distributeEvents(int nevents_to_process) {
 	mt19937                     generator(randomDevice());
 	uniform_real_distribution<> randomDistribution(0, 1);
 
+	// Weights in the run-weights file are relative, not required to sum to 1. Compute the total
+	// once and scale each draw by it, so non-normalized weights distribute events correctly.
+	double totalWeight = 0;
+	for (const auto& weight : runWeights) { totalWeight += weight.second; }
+	if (totalWeight <= 0) {
+		log->error(ERR_EVENTDISTRIBUTIONFILENOTFOUND,
+		           "Run weights sum to ", totalWeight, " (must be > 0). Check your run weights file.");
+		return;
+	}
+
 	// For each event, select a run by comparing a random draw to the cumulative weight intervals.
-	// This assumes runWeights values represent fractions or relative weights normalized to sum to 1.
 	for (int i = 0; i < nevents_to_process; i++) {
-		double randomNumber = randomDistribution(generator);
+		double randomNumber = randomDistribution(generator) * totalWeight;
 
 		double cumulativeWeight = 0;
 		for (const auto& weight : runWeights) {