Avoid work-stealing in bytecode compilation (#4004)
## Summary
Avoid using work-stealing Tokio workers for bytecode compilation,
favoring instead dedicated threads. Tokio's work-stealing does not
really benefit us because we're spawning Python workers and scheduling
tasks ourselves — we don't want Tokio to re-balance our workers. Because
we're doing scheduling ourselves and compilation is a primarily
compute-bound task, we can also create dedicated runtimes for each
worker and avoid some synchronization overhead.
This is part of a general desire to avoid relying on Tokio's
work-stealing scheduler and be smarter about our workload. In this case
we already had the custom scheduler in place, Tokio was just getting in
the way (though the overhead is very minor).
## Test Plan
This improves performance by ~5% on my machine.
```
$ hyperfine --warmup 1 --prepare "target/profiling/uv-dev clear-compile .venv" "target/profiling/uv-dev compile .venv" "target/profiling/uv-dev-dedicated compile .venv"
Benchmark 1: target/profiling/uv-dev compile .venv
Time (mean ± σ): 1.279 s ± 0.011 s [User: 13.803 s, System: 2.998 s]
Range (min … max): 1.261 s … 1.296 s 10 runs
Benchmark 2: target/profiling/uv-dev-dedicated compile .venv
Time (mean ± σ): 1.220 s ± 0.021 s [User: 13.997 s, System: 3.330 s]
Range (min … max): 1.198 s … 1.272 s 10 runs
Summary
target/profiling/uv-dev-dedicated compile .venv ran
1.05 ± 0.02 times faster than target/profiling/uv-dev compile .venv
$ hyperfine --warmup 1 --prepare "target/profiling/uv-dev clear-compile .venv" "target/profiling/uv-dev compile .venv" "target/profiling/uv-dev-dedicated compile .venv"
Benchmark 1: target/profiling/uv-dev compile .venv
Time (mean ± σ): 3.631 s ± 0.078 s [User: 47.205 s, System: 4.996 s]
Range (min … max): 3.564 s … 3.832 s 10 runs
Benchmark 2: target/profiling/uv-dev-dedicated compile .venv
Time (mean ± σ): 3.521 s ± 0.024 s [User: 48.201 s, System: 5.392 s]
Range (min … max): 3.484 s … 3.566 s 10 runs
Summary
target/profiling/uv-dev-dedicated compile .venv ran
1.03 ± 0.02 times faster than target/profiling/uv-dev compile .venv
```
