Add Const annotation for Enzyme

[penelopeysm]

This makes Enzyme happy enough to start an attempt at doing AD. It still fails (on DPPL 0.37), but that's already been reported upstream.

Once it starts passing, though, I'll need to benchmark it to see whether the closure is better or not.

[github-actions]

Benchmark Report for Commit 361a639

Computer Information

Julia Version 1.11.5
Commit 760b2e5b739 (2025-04-14 06:53 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: Linux (x86_64-linux-gnu)
  CPU: 4 × AMD EPYC 7763 64-Core Processor
  WORD_SIZE: 64
  LLVM: libLLVM-16.0.6 (ORCJIT, znver3)
Threads: 1 default, 0 interactive, 1 GC (on 4 virtual cores)

Benchmark Results

|                 Model | Dimension |  AD Backend |      VarInfo Type | Linked | Eval Time / Ref Time | AD Time / Eval Time |
|-----------------------|-----------|-------------|-------------------|--------|----------------------|---------------------|
| Simple assume observe |         1 | forwarddiff |             typed |  false |                  9.0 |                 1.6 |
|           Smorgasbord |       201 | forwarddiff |             typed |  false |                702.0 |                40.5 |
|           Smorgasbord |       201 | forwarddiff | simple_namedtuple |   true |                437.8 |                50.3 |
|           Smorgasbord |       201 | forwarddiff |           untyped |   true |               1000.2 |                35.9 |
|           Smorgasbord |       201 | forwarddiff |       simple_dict |   true |               6989.9 |                27.4 |
|           Smorgasbord |       201 | reversediff |             typed |   true |               1551.6 |                27.1 |
|           Smorgasbord |       201 |    mooncake |             typed |   true |               1067.9 |                 4.2 |
|    Loop univariate 1k |      1000 |    mooncake |             typed |   true |               6164.0 |                 3.8 |
|       Multivariate 1k |      1000 |    mooncake |             typed |   true |               1027.9 |                 8.9 |
|   Loop univariate 10k |     10000 |    mooncake |             typed |   true |              69163.8 |                 3.5 |
|      Multivariate 10k |     10000 |    mooncake |             typed |   true |               8853.4 |                10.0 |
|               Dynamic |        10 |    mooncake |             typed |   true |                142.1 |                12.5 |
|              Submodel |         1 |    mooncake |             typed |   true |                 13.6 |                 6.1 |
|                   LDA |        12 | reversediff |             typed |   true |               1281.1 |                 4.2 |

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.94%. Comparing base (3af63d5) to head (361a639).
Report is 1 commits behind head on breaking.

Additional details and impacted files

@@            Coverage Diff            @@
##           breaking     #957   +/-   ##
=========================================
  Coverage     82.94%   82.94%           
=========================================
  Files            38       38           
  Lines          4068     4068           
=========================================
  Hits           3374     3374           
  Misses          694      694           

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