docs: note on non-transparent wrapper evaluators under GraphEvaluator

[ptomecek]

Summary

Adds a docs subsection to the "Write a custom evaluator" how-to (docs/wiki/how-to/Cache-Results.md) covering a subtle footgun when a non-transparent wrapper evaluator (one that reshapes, replaces, or publishes the result) is combined with GraphEvaluator.

GraphEvaluator re-drives both __deps__ resolution and child-node evaluations through the same evaluator stack. A wrapper that fires on every invocation therefore:

• runs on the GraphDepList payload of __deps__ (not a model result) — reshaping logic such as .df access crashes there; and
• side-effects on dependency results it should not own — double-publishing and changing cache identity so it no longer matches the true compute.

What the note says

• Gate the wrapper on context.fn == "__call__" so it never acts on __deps__ (includes a small PublishingEvaluator example).
• Scope the override to specific models / model_types to avoid firing on dependencies — but note this is not sufficient on its own, because a model still re-drives its own __deps__, so the fn == "__call__" guard is always required.
• Points at the built-in DryRunEvaluator as a worked example (passes through on __deps__, uses a re-entry guard for child drives).

The findings were verified empirically against a GraphEvaluator stack.

Notes

• Docs-only change; no code changes.
• mdformat --check and codespell pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.48%. Comparing base (6a788a6) to head (6328113).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

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  Coverage   93.48%   93.48%           
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  Files         176      176           
  Lines       20327    20327           
  Branches     1350     1350           
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  Hits        19002    19002           
  Misses       1052     1052           
  Partials      273      273           

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[github-actions]

Test Results

    1 files  ±0      1 suites  ±0   3m 8s ⏱️ +8s
1 328 tests ±0  1 326 ✅ ±0  2 💤 ±0  0 ❌ ±0 
1 334 runs  ±0  1 332 ✅ ±0  2 💤 ±0  0 ❌ ±0 

Results for commit 6328113. ± Comparison against base commit 6a788a6.

♻️ This comment has been updated with latest results.