Fix independent LF resonance gun RNG seeding

[mjkim525]

Summary

The LF resonance gun uses a separate Pythia random-number generator (RNG) for mass sampling and decays, which was not explicitly seeded. Initialize it with a
positive seed drawn from ROOT to avoid unintended reuse of the default random sequence.

• Preserve the existing gRandom->SetSeed(0) behavior and explicit decayer configuration overrides.
• Log the effective gun seed once during construction and warn if seeding is disabled or the seed is nonpositive.

Details

Previously, the gun's separate RandomNumberGenerator was left at its default initialization.
Independently launched jobs could therefore reuse the same random sequence, producing repeated resonance-mass sequences and artificial structures in the combined daughter invariant-mass spectrum, as illustrated in Figure 1 (from the recent production for 23 periods in this JIRA ticket).

Figure 2 compares injected Xi(1820) daughter invariant-mass distributions before (blue) and after (red) this change, using 20,000 injected resonances per version.

In this local generator-level comparison, the updated sample no longer exhibits repeated mass sequences across jobs and more smoothly samples the configured Pythia

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