CodSpeed runner hangs when using divan's `threads` feature

[MichaReiser]

Salsa's parallel benchmarks are extremely noisy. I hoped to reduce the noise by using divan's threads feature (salsa-rs/salsa#1272). Unfortunately, CodSpeed hangs.

Codex analysis:

Yes. I reproduced a real race condition in CodSpeed’s Divan integration:

1. Every Divan worker calls add_benchmark_timestamps concurrently. Source
2. CodSpeed declares its shared instrumentation handle Sync without synchronization. Source
3. Workers consequently mutate the same FIFO buffer and write message headers and bodies separately, corrupting the protocol. Source

Using a local mock CodSpeed runner:

1 worker: completed successfully
4 workers: Corrupt command tag: 20 → hangs

CodSpeed needs either synchronized access to its instrumentation handle or instrumentation emitted by only one worker.