diagnostics_channel: add threadpool work event

[IlyasShabi]

This PR aims to add support for experimental threadpool.work diagnostics channel.

The event exposes:

• type: kind of threadpool work
• enqueued: submission timestamp
• started: execution start timestamp
• ended: execution end timestamp

These timestamps allow diagnostics tools to distinguish queue time from native execution time. Cancelled work reports started and ended as null.

The channel and subscriber state are cached per Environment so the unsubscribed path remains minimal.

Benchmark

Unsubscribed configurations showed no significant regressions. Most subscribed configurations were also within noise; zlib at n=10000 measured a 3.48% regression on local machine (will share the onces from CI)

[nodejs-github-bot]

Review requested:

• @nodejs/performance

[mcollina]

while this is a good generic API, I'm not convinced. Shouldn't we add one for each usage?

cc @bengl @Qard ?

[IlyasShabi]

@mcollina You mean separate channels named threadpool.work.<type>? This is also a valid approach and would let consumers subscribe only to relevant work and remove type from the payload.

[codecov]

Codecov Report

❌ Patch coverage is 88.70968% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.19%. Comparing base (b3fb344) to head (946f372).
⚠️ Report is 29 commits behind head on main.

Files with missing lines	Patch %	Lines
src/threadpoolwork-inl.h	88.57%	0 Missing and 4 partials ⚠️
src/env.cc	81.25%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65982      +/-   ##
==========================================
+ Coverage   90.16%   90.19%   +0.02%     
==========================================
  Files         771      771              
  Lines      265434   265513      +79     
  Branches    50450    50478      +28     
==========================================
+ Hits       239332   239468     +136     
+ Misses      17045    17003      -42     
+ Partials     9057     9042      -15     

Files with missing lines	Coverage Δ
src/env.h	97.36% <100.00%> (+0.14%)	⬆️
src/node_diagnostics_channel.cc	86.66% <100.00%> (+3.01%)	⬆️
src/node_diagnostics_channel.h	100.00% <100.00%> (+42.85%)	⬆️
src/node_internals.h	81.66% <100.00%> (+1.30%)	⬆️
src/env.cc	82.15% <81.25%> (+0.05%)	⬆️
src/threadpoolwork-inl.h	91.22% <88.57%> (-4.23%)	⬇️

... and 46 files with indirect coverage changes

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