ReconParticleDriver: make longitudinal beam size configurable via setBeamSigmaZ

beamSize[0] (tracking-frame X = detector Z) is the longitudinal width used as the target-constraint vertex covariance. Previously only sigmaX/sigmaY had setters and beamSize[0] stayed hard-wired to 1 um, over-constraining the target-constrained vertex Z and skewing post-fit track momenta. Exposes it as the <beamSigmaZ> steering knob.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: mgignac

recon/src/main/java/org/hps/recon/particle/ReconParticleDriver.java

@@ -285,6 +285,16 @@ public void setBeamSigmaY(double sigmaY) {
         beamSize[2] = sigmaY; // The beamsize array is in the tracking frame HPS Y => TRACK Z
     }
 
+    /**
+     * Sets the beam size sigma in the z-direction.
+     *
+     * @param sigmaZ - The standard deviation of the beam width in the
+     * z-direction (beam/target longitudinal spread).
+     */
+    public void setBeamSigmaZ(double sigmaZ) {
+        beamSize[0] = sigmaZ; // The beamsize array is in the tracking frame HPS Z => TRACK X
+    }
+
     /**
      * Sets the beam position in the z-direction in mm.
      *