fix: High memory usage from sample-level result accumulators

[akrivi]

Previously, each thread allocated sample-level accumulators sized for the full Monte Carlo sample count. For sample-based result specs like ShortfallSamples, FlowSamples, StorageEnergySamples, GeneratorAvailability, this caused memory usage to scale as O(regions x timesteps x samples x threads).

This PR changes threaded execution so sample-based result accumulators are partitioned by each worker’s assigned sample range.

Example for 3 threads:

Before, each thread allocated the whole sample matrix:

Thread 1: [ total number of samples] 

Thread 2: [ total number of samples]

Thread 3: [ total number of samples]

Now, samples are split into ranges:

Thread 1: [ 1/3 (total number of samples)] 

Thread 2: [ 1/3 (total number of samples)]

Thread 3: [ 1/3 (total number of samples)]

Benchmarks

System: Guam 2028, 13 regions, 8760 timestamps, hourly resolution
Simulation: Run on HPC, using standard nodes (104 cores, 250 GB)
Result: ShortfallSamples()

1000 MC Samples

threads	Elapsed time(s) - main	Elapsed time(s) - PR	Max RSS(GB) - main	Max RSS(GB) - PR
1	23.61	23.75	1.55	2.41
2	13.04	12.96	2.37	2.38
4	8.28	8.16	4.09	2.38
8	6.68	5.24	7.47	2.41
16	6.84	3.87	14.31	2.43
24	7.39	3.48	21.14	2.46
32	9.59	3.28	27.99	2.49
48	11.16	3.06	41.69	2.62
64	14.53	2.97	55.28	2.61
80	15.04	2.96	69.01	2.75
96	21.48	3.06	82.61	2.79

10000 MC Samples

threads	Elapsed time(s) - main	Elapsed time(s) - PR	Max RSS(GB) - main	Max RSS(GB) - PR
1	210.73	208.58	9.19	17.68
2	109.29	108.45	17.67	17.65
4	61.39	56.88	34.64	17.64
8	46.96	33.21	68.58	17.66
16	43.08	18.88	136.53	17.69
24	51.95	13.68	204.37	17.77

[codecov-commenter]

Codecov Report

❌ Patch coverage is 98.59155% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 82.30%. Comparing base (c793303) to head (b113d5e).

Files with missing lines	Patch %	Lines
PRASCore.jl/src/Simulations/Simulations.jl	97.14%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #109      +/-   ##
==========================================
- Coverage   83.16%   82.30%   -0.86%     
==========================================
  Files          45       45              
  Lines        2429     2475      +46     
==========================================
+ Hits         2020     2037      +17     
- Misses        409      438      +29     

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