wall profiler: use thread-local for the active profiler

[szegedi]

Summary

• Replace the ProtectedProfilerMap (an unordered_map<Isolate*, WallProfiler*> shielded by an atomic-exchange dance so the SIGPROF handler could read it lock-free) with a thread_local WallProfiler* t_active_profiler. In Node's threading model each V8 isolate is pinned to a single thread, so a per-thread pointer captures exactly the fact the signal handler needs without a map, or atomics, or copy-on-write churn on every start/stop. This is the same approach we adopted in the equivalent Node.js core port (Attach values to CPU samples nodejs/node#63163).
• Keep a separate ActiveProfilers registry (plain std::mutex + std::unordered_set<WallProfiler*>) for the one cross-thread path that survives: the main thread gathering worker-thread CPU while building a profile. That path has no signal-safety requirement, so the concurrency machinery was eliminated from there as well except a simple mutex.
• Dispose loses its removeFromMap parameter — it now always removes itself from the registry and clears the TLS pointer. CreateV8CpuProfiler checks the TLS to refuse a duplicate start instead of relying on the map's emplace returning false.

This simplifies the SIGPROF handler, the start/stop hot path no longer copies a map under a mutex. This also solves the problem of one thread's SIGPROF handler not being able to find its own map while another is added to it, which would've lost us processing of samples.

Test plan

• npm run build — clean
• npm test — 88/88 passing, including Worker Threads ("should work", "should not crash when worker is terminated") which exercises cross-thread CPU gather and destroy-during-profiling
• npm run format — clean
• CI green

[github-actions]

Overall package size

Self size: 2 MB
Deduped: 2.36 MB
No deduping: 2.36 MB

Dependency sizes

| name | version | self size | total size | |------|---------|-----------|------------| | source-map | 0.7.6 | 185.63 kB | 185.63 kB | | pprof-format | 2.2.1 | 163.06 kB | 163.06 kB | | node-gyp-build | 4.8.3 | 13.81 kB | 13.81 kB |

_{🤖 This report was automatically generated by heaviest-objects-in-the-universe}