add VariableType enum

[hardik-xi11]

A start for #327
Only added the enum for identifying nodes in this commit. Next we can use the implementation so the samplers can optionally skip these nodes during MCMC for better performance.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.63%. Comparing base (927f61f) to head (21fe7f6).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #443      +/-   ##
==========================================
+ Coverage   83.56%   83.63%   +0.06%     
==========================================
  Files          27       27              
  Lines        3949     3965      +16     
==========================================
+ Hits         3300     3316      +16     
  Misses        649      649              

Flag	Coverage Δ
callbacks	17.48% <68.75%> (+0.77%)	⬆️
compilation_model	34.17% <100.00%> (+0.88%)	⬆️
compilation_source_gen	34.86% <87.50%> (+0.83%)	⬆️
distributed_sampling	29.56% <75.00%> (+0.77%)	⬆️
elementary	23.60% <81.25%> (+0.85%)	⬆️
frontend	48.16% <87.50%> (+0.77%)	⬆️
gibbs	28.34% <87.50%> (+0.26%)	⬆️
graphs	18.73% <75.00%> (+0.25%)	⬆️
inference_chains	22.34% <87.50%> (+0.26%)	⬆️
inference_hmc	22.51% <87.50%> (+0.26%)	⬆️
inference_marginalization	25.39% <68.75%> (+0.17%)	⬆️
inference_mh	23.04% <75.00%> (+0.23%)	⬆️
log_density	35.83% <87.50%> (+0.26%)	⬆️
model_operations	31.19% <87.50%> (+0.84%)	⬆️
parallel_sampling	19.38% <75.00%> (+0.22%)	⬆️

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[github-actions]

Benchmark Results

Julia 1.11.9 on AMD EPYC 7763 64-Core Processor (Linux)

Ratio = JuliaBUGS/Stan (lower is better for JuliaBUGS)

Model	Stan Params	JBUGS Params	LD Ratio	Grad Ratio
rats	65	65	0.35x	1.70x
pumps	12	12	0.61x	1.90x
bones	13	33	0.69x	4.58x
oxford	244	244	1.70x	4.55x
epil	303	303	0.53x	1.65x
lsat	1006	1006	0.67x	1.96x
schools	133	133	0.24x	1.57x
beetles	2	2	1.39x	4.85x
air	5	5	0.52x	1.42x

Note: Stan benchmarks use hand-optimized Stan models, not direct BUGS translations. Comparison is illustrative only.