test: add pyhl::Runtime integration tests

[danbugs]

Summary

Adds 13 integration tests for the pyhl::Runtime library API, exercising the programmatic interface that downstream consumers use:

• Basic execution: hello world, pandas import
• Exit code propagation: sys.exit(0), sys.exit(42), unhandled exception
• Hermetic execution: verifies globals don't leak between run_code() calls, repeated stable runs
• Filesystem via Preopen: write+read, listdir, mkdir+stat — verifies host-side files match
• Network policy: AllowList permits allowed host, blocks unlisted host, disabled by default

All tests self-skip gracefully if pyhl isn't set up or no hypervisor is available (same pattern as snapshot_roundtrip.rs).

Test plan

• All 13 tests pass locally with cargo test --test pyhl_runtime -- --test-threads=1
• cargo clippy clean
• cargo fmt clean
• Tests self-skip on machines without /dev/kvm or pyhl setup

[github-actions]

Linux Benchmarks

Details

Benchmark suite	Current: c64d5c8	Previous: 86d2609	Ratio
hello_world (median)	20 ms	20 ms	1
pandas (median)	100 ms	110 ms	0.91
density (per VM)	7 MB	7 MB	1
snapshot (disk)	385 MiB	385 MiB	1

This comment was automatically generated by workflow using github-action-benchmark.

[github-actions]

Windows Benchmarks

Details

Benchmark suite	Current: c64d5c8	Previous: 86d2609	Ratio
hello_world (median)	225 ms	245 ms	0.92
pandas (median)	658 ms	725 ms	0.91
density (per VM)	6 MB	6 MB	1
snapshot (disk)	392 MiB	392 MiB	1

This comment was automatically generated by workflow using github-action-benchmark.