feat(exec-harness): declare the benchmarked command's pid

[lvaroqui]

Make exec-harness declare the pid of the command it benchmarks, so its own spawning cost can be left out of simulation results.

Since #531, exec-harness turns instrumentation on in its own process and spawns the command, which inherits that state across fork and exec. The dumped part therefore also holds the harness's cost of spawning the command and waiting for it. That is a fixed overhead on every benchmark, about 207k instructions per command locally.

Changes:

• instrument-hooks binding: new set_executed_benchmark_for_pid(pid, uri). set_executed_benchmark keeps its behavior and delegates to it.
• exec-harness: spawns the command and passes its pid, instead of calling Command::status(). In memory mode the pid only travels to the runner's FIFO, where nothing reads it for exec-harness.
• Submodule bump: feat(valgrind): declare the pid a benchmark ran in instrument-hooks#32 writes desc: Benchmark pid: <pid> in the part when the pid is not the caller's. This bump also brings the thread-safe C API exports and the callgrind_toggle_collect helper.

Checked locally under the patched valgrind, with the runner's simulation flags on sh -c '/bin/true; /bin/true; :':

part: 2
desc: Spawned pid: 289126
desc: Benchmark pid: 289126
desc: Trigger: Client Request: exec_harness::true_twice

Still to do before this is ready:

• Bump the valgrind pin to 0codspeed8 once feat(callgrind): add CALLGRIND_ADD_DESC client request valgrind-codspeed#43 is released. Until then the request is ignored with a warning and results keep the harness's cost.
• Repoint the submodule once feat(valgrind): declare the pid a benchmark ran in instrument-hooks#32 merges.
• The backend support for leaving out the declared launcher cost isn't released yet.

Closes COD-3722

[codspeed]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 31 untouched benchmarks
⏩ 6 skipped benchmarks¹

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_{Comparing cod-3722-ignore-process-spawning-overhead-in-exec-harness-simulation (345f77b) with spike/cod-3440-memtrack-musl (3a766f1)}

Footnotes

1. 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩