fix: correct test fd and convert HostSocket to struct

[danbugs]

Summary

• Fixes net_bind_denied_without_listen_ports test to use the actual fd returned by net_socket instead of hardcoded fd:0 — the test previously passed only because the permission check fires before fd lookup
• Converts HostSocket from a single-variant enum to a plain struct with named fields, simplifying all match sites to direct field access

Test plan

• cargo test --lib passes (51 tests)
• cargo clippy --lib clean
• cargo fmt clean

[github-actions]

Linux Benchmarks

Details

Benchmark suite	Current: 65babe2	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: 65babe2	Previous: 86d2609	Ratio
hello_world (median)	260 ms	245 ms	1.06
pandas (median)	883 ms	725 ms	1.22
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.