Fix thread safety of debug raw value accumulator (#1337)

[penelopeysm]

Closes #1337

[github-actions]

Benchmark Report

• this PR's head: 04682e150b82f2b25dc8b0726a831d89378a978b
• base branch: a1e8f06f4e38c61ba9364201d44b2f6c1950be41

Computer Information

Julia Version 1.11.9
Commit 53a02c0720c (2026-02-06 00:27 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

┌───────────────────────┬───────┬─────────────┬────────┬───────────────────────────────┬────────────────────────────┬─────────────────────────────────┐
│                       │       │             │        │       t(eval) / t(ref)        │     t(grad) / t(eval)      │        t(grad) / t(ref)         │
│                       │       │             │        │ ─────────┬──────────┬──────── │ ───────┬─────────┬──────── │ ──────────┬───────────┬──────── │
│                 Model │   Dim │  AD Backend │ Linked │     base │  this PR │ speedup │   base │ this PR │ speedup │      base │   this PR │ speedup │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│               Dynamic │    10 │    mooncake │   true │   293.76 │   293.15 │    1.00 │   6.58 │    8.03 │    0.82 │   1932.08 │   2354.42 │    0.82 │
│                   LDA │    12 │ reversediff │   true │  3271.74 │  2934.54 │    1.11 │   2.55 │    3.34 │    0.76 │   8355.42 │   9798.55 │    0.85 │
│   Loop univariate 10k │ 10000 │    mooncake │   true │ 29280.88 │ 30831.57 │    0.95 │   7.38 │    6.55 │    1.13 │ 216051.80 │ 201957.15 │    1.07 │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│    Loop univariate 1k │  1000 │    mooncake │   true │  4152.59 │  3138.28 │    1.32 │   5.27 │    6.40 │    0.82 │  21889.12 │  20075.26 │    1.09 │
│      Multivariate 10k │ 10000 │    mooncake │   true │ 44541.07 │ 31630.24 │    1.41 │   6.74 │   10.01 │    0.67 │ 300230.46 │ 316596.51 │    0.95 │
│       Multivariate 1k │  1000 │    mooncake │   true │  3869.31 │  3394.53 │    1.14 │   8.19 │    9.34 │    0.88 │  31671.62 │  31688.73 │    1.00 │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│ Simple assume observe │     1 │ forwarddiff │  false │     0.88 │     0.88 │    1.00 │  11.44 │   10.52 │    1.09 │     10.05 │      9.23 │    1.09 │
│           Smorgasbord │   201 │ forwarddiff │  false │   956.99 │   954.32 │    1.00 │  77.17 │   70.98 │    1.09 │  73853.04 │  67733.42 │    1.09 │
│           Smorgasbord │   201 │      enzyme │   true │  1298.14 │  1325.83 │    0.98 │   4.43 │    4.78 │    0.93 │   5752.21 │   6338.05 │    0.91 │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│           Smorgasbord │   201 │ forwarddiff │   true │  1313.86 │  1294.91 │    1.01 │  69.55 │   68.04 │    1.02 │  91384.46 │  88110.78 │    1.04 │
│           Smorgasbord │   201 │    mooncake │   true │  1305.51 │  1310.37 │    1.00 │   4.55 │    4.72 │    0.96 │   5936.69 │   6183.67 │    0.96 │
│           Smorgasbord │   201 │ reversediff │   true │  1284.39 │  1307.38 │    0.98 │ 123.62 │  135.17 │    0.91 │ 158770.21 │ 176715.97 │    0.90 │
├───────────────────────┼───────┼─────────────┼────────┼──────────┼──────────┼─────────┼────────┼─────────┼─────────┼───────────┼───────────┼─────────┤
│              Submodel │     1 │    mooncake │   true │     0.87 │     0.88 │    0.99 │  29.28 │   26.78 │    1.09 │     25.57 │     23.56 │    1.09 │
└───────────────────────┴───────┴─────────────┴────────┴──────────┴──────────┴─────────┴────────┴─────────┴─────────┴───────────┴───────────┴─────────┘

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.93%. Comparing base (a1e8f06) to head (04682e1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/accumulators/vnt.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1338   +/-   ##
=======================================
  Coverage   77.93%   77.93%           
=======================================
  Files          50       50           
  Lines        3585     3585           
=======================================
  Hits         2794     2794           
  Misses        791      791           

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