Make log density function generation on-demand

[sunxd3]

This PR introduces on-demand generation of log density functions for JuliaBUGS models.

The skip_source_generation parameter has been removed from compile() and BUGSModel(). Instead, log density functions are now generated on-demand when set_evaluation_mode(model, UseGeneratedLogDensityFunction()) is called. All models start with UseGraph() mode.

[github-actions]

JuliaBUGS.jl documentation for PR #416 is available at:
https://TuringLang.github.io/JuliaBUGS.jl/previews/PR416/

[coveralls]

Pull Request Test Coverage Report for Build 19886678447

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 19 of 22 (86.36%) changed or added relevant lines in 4 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.007%) to 83.867%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
JuliaBUGS/src/serialization.jl	0	1	0.0%
JuliaBUGS/src/model/bugsmodel.jl	16	18	88.89%

Files with Coverage Reduction	New Missed Lines	%
JuliaBUGS/src/serialization.jl	2	0.0%

Totals
Change from base Build 19866622348:	-0.007%
Covered Lines:	3301
Relevant Lines:	3936

---

💛 - Coveralls

[github-actions]

Benchmark Results

Julia 1.11.7 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	1.71x
pumps	12	12	0.64x	1.85x
bones	13	33	0.67x	4.36x
oxford	244	244	1.73x	4.37x
epil	303	303	0.51x	1.60x
lsat	1006	1006	0.65x	2.01x
schools	133	133	0.23x	1.45x
beetles	2	2	1.16x	4.09x
air	5	5	0.54x	1.45x

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

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.86%. Comparing base (5208959) to head (79b350c).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
JuliaBUGS/src/model/bugsmodel.jl	88.88%	2 Missing ⚠️
JuliaBUGS/src/serialization.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #416      +/-   ##
==========================================
- Coverage   83.87%   83.86%   -0.01%     
==========================================
  Files          31       31              
  Lines        3944     3936       -8     
==========================================
- Hits         3308     3301       -7     
+ Misses        636      635       -1     

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