chore(metrics): replace vendored JSF PRNG with inline SplitMix64 (#700)

[leoparente]

Closes #700.

What

Replaces the vendored 3rd/rng/jsf.h (178-line Bob Jenkins "Small Fast" PRNG) with a ~6-line inline SplitMix64 in AbstractMetricsManager.h, and deletes the 3rd/rng/ directory.

JSF was used in exactly one spot — the deep-sampling gate:

_deep_sampling_now.store((_rng() % 100U < _deep_sample_rate), ...);

i.e. "roll 0–99, deep-sample this event if the roll is under deep_sample_rate." The sampling rate is unchanged; only which events get deep-sampled differs, and nothing depends on the specific set (default deep_sample_rate=100 means the RNG is never even called).

Why SplitMix64 instead of the proposed pcg-cpp

The issue suggested the pcg-cpp conan package. I went a different way after weighing the options:

• pcg-cpp adds a third-party dependency to remove a dependency-free header — the wrong direction for "reduce maintained code" — and its idiomatic seeding seed_seq_from<std::random_device> is exactly the footgun that caused RDRAND failure on Arista switch #109 (below).
• std::mt19937 works but carries ~2.5 KB of state per handler instance for a 1-in-100 coin flip.
• Inline SplitMix64 removes the whole 3rd/rng/ dir with no new dependency, 8-byte state, and BigCrush-grade quality. We "own" ~6 auditable lines instead of 178.

Invariant preserved (#109)

#109 ("RDRAND failure on Arista switch") was a startup crash: the pre-JSF randutils RNG auto-seeded from std::random_device → RDRAND, which faults and throws on some hardware. PR #110 fixed it by switching to a fixed-seeded PRNG. SplitMix64 here is seeded from a fixed constant and never from hardware entropy, preserving that fix. A prominent comment documents the constraint so it isn't "improved" into entropy seeding later.

Build wiring

• Delete 3rd/rng/ + its add_subdirectory + the rng link in visor-core.
• Fix a latent copy-paste bug in 3rd/timer/CMakeLists.txt: it called target_include_directories(rng ...) instead of (timer ...), so timer.hpp's include dir was being attached to the rng target and only reached visor-core transitively through it. Removing rng would have broken <timer.hpp>; pointing it at timer makes the timer target self-contained.
• Update the NOMINMAX comment (it cited jsf.h's static min()/max()); the define stays for general Windows hygiene.

Testing

unit-tests-visor-core (which includes the sampling/metrics tests) passes — 1153 assertions, 21 test cases.

🤖 Generated with Claude Code

[github-actions]

LCOV of commit 6531af3 during Debug Builds #162

  lines......: 82.4% (14103 of 17109 lines)
  functions..: 72.8% (1424 of 1955 functions)
  branches...: no data found

Files changed coverage rate: n/a

Full coverage report

[leoparente]

@codex review

[Copilot]

Pull request overview

This PR removes the vendored JSF PRNG implementation and replaces its sole usage (deep-sampling gate in metrics) with a small inline SplitMix64 PRNG seeded from a fixed constant, while also cleaning up the CMake wiring to drop the removed rng interface target and ensure the timer interface target is self-contained.

Changes:

• Replace #include <jsf.h> + jsf32 usage with an inline fixed-seed SplitMix64 generator for deep-sampling decisions.
• Remove the 3rd/rng/ directory and unlink/unsubdir it from the build.
• Fix 3rd/timer CMake to attach include dirs to the timer target (not the deleted rng target) and update WIN32 NOMINMAX comment to be general-purpose.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/CMakeLists.txt	Stops linking visor-core against the removed rng interface target.
src/AbstractMetricsManager.h	Introduces inline fixed-seed SplitMix64 PRNG and removes dependency on jsf.h.
CMakeLists.txt	Updates WIN32 NOMINMAX comment to reflect broader rationale (not JSF-specific).
3rd/timer/CMakeLists.txt	Fixes include-directory attachment to the correct timer target (prevents breakage after rng removal).
3rd/rng/jsf.h	Deletes vendored JSF PRNG header.
3rd/rng/CMakeLists.txt	Deletes the rng interface target definition.
3rd/CMakeLists.txt	Removes add_subdirectory(rng) from third-party build wiring.

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[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: 0a3c86889c

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