docs(performance): separate pre-#354 cold baseline from warm post-fix measurement

[koriym]

Follow-up to #135. The existing "Measured at production scale" table was a single cold pre-fix observation; it's now explicitly labeled as historical baseline. Adds a distinct section for the post-ray-di/Ray.Di#354 measurement (paired before/after + three-way, warm OPcache SHM, same production app), kept in its own table so cold and warm numbers are never compared or ratioed against each other.

Data and method: #135 (comment), raw per-cycle data at https://gist.github.com/koriym/db1c3b8bccd8c7c6bc18d4a2a51c622c

Summary by CodeRabbit

• Documentation
  • Clarified that historical performance figures represent a single cold-process observation, not a controlled comparison.
  • Added paired warm-cache measurements showing approximately 34% improvement in initialization and deserialization performance.
  • Added warm comparison results for reflection, serialization, and compiled approaches, with guidance not to compare them directly with historical cold-run figures.
  • Clarified request-based runtime behavior and cost amortization in long-lived workers.
  • Added guidance for verifying that Xdebug is inactive during benchmarks and corrected earlier measurements affected by Xdebug.

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📝 Walkthrough

Walkthrough

The performance documentation clarifies request-scoped costs, verifies Xdebug state, labels historical cold measurements, and adds controlled warm-OPcache results after the unserialize() fix.

Changes

Performance measurement documentation

Layer / File(s)	Summary
Runtime and benchmark guidance docs/performance.md	The document states which costs recur per request and explains how to verify Xdebug state in the benchmarked process.
Measurement baselines docs/performance.md	The historical production measurements are labeled as cold, pre-fix, and indicative. The document adds paired warm-OPcache results, improvement counts, comparison timings, ratios, and measurement-scope cautions.

Priority: ⬇️ Low

Estimated code review effort: 1 (Trivial) | ~5 minutes

Change: Other

Merge Risk: ⚪ Minimal · up to cc0ae

The updated performance documentation is ready to merge.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main documentation change: it separates the historical pre-#354 cold baseline from the warm post-fix measurement.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (741b185) to head (cc0ae2a).

Additional details and impacted files

@@             Coverage Diff             @@
##                 1.x      #145   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
  Complexity       119       119           
===========================================
  Files             12        12           
  Lines            302       302           
===========================================
  Hits             302       302           

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Actionable comments posted: 1

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Inline comments:
In `@docs/performance.md`:
- Around line 151-152: The performance table’s reflection/serialize ratio is
inconsistent with the displayed medians; either change 42.6× to 42.5× or
explicitly note that it was calculated from unrounded medians from
ray-di/Ray.Compiler#135.

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