Feature egspp xoshiro256pp

[mainegra]

● Add xoshiro256++ RNG to egs++ library

Summary

• Adds EGS_Xoshiro256pp class to egs_rndm.cpp implementing the xoshiro256++ generator (Blackman & Vigna, 2019)
• Adds virtual EGS_I64 getUInt64() to the EGS_RandomGenerator base class (returns 0 by default, enabling safe fallback in user code)
• Activated via type = xoshiro256++ in the rng definition input block

Generator properties

Property	Value
State	256-bit (4 × uint64_t), period 2²⁵⁶ − 1
Seeding	SplitMix64 expansion from two integer seeds
Parallel streams	jump() skips 2¹²⁸ states per sequence step — provably non-overlapping
Standard output	fillArray(): 53-bit doubles, τ_max ≈ 36.7 mfp
Extended output	getUInt64(): raw 64-bit integer, τ_max ≈ 44.4 mfp
State persistence	storePrivateState / setPrivateState — .egsdat compatible

The getUInt64() method is intended for free-path sampling: the caller computes
dpmfp = -log((double)(uint64_t)r) + 64·ln2. User codes whose RNG does not support
this method receive the base-class return value of 0 and can fall back to the standard
-log(1 − U) path.

Motivation

The high-resolution RANMAR (48-bit, τ_max ≈ 33.3 mfp) produces an artifact in
deep-penetration calculations: photon interaction sites are capped at τ_max, potentially creating a non-physical drop in depth-dose and buildup-factor curves. xoshiro256++ extends the reliable range and provides a
higher-quality generator with better-characterised parallel-stream properties than
RANMAR's seed-increment approach.

Usage: Place the following block in the input file

 :start rng definition:
      type = xoshiro256++
      initial seeds = 1802 9373
  :stop rng definition:

Test plan

• Confirm type = xoshiro256++ accepted in input without error
• Confirm ranmar behaviour unchanged (default RNG and defaultRNG() untouched)
• Run existing regression tests with xoshiro256++ and verify results agree with ranmar within statistics
• Confirm .egsdat state round-trip (store → restore → identical subsequent output)
• Confirm parallel jobs produce non-overlapping streams