Add CodSpeed performance benchmarks

[codspeed-hq]

Summary

This PR integrates CodSpeed for continuous performance tracking of pytest-socket's core utility functions.

Changes

Benchmark tests (tests/test_benchmarks.py)

12 benchmarks covering the plugin's hot paths:

• is_ipaddress -- IPv4, IPv6, and invalid address validation
• host_from_address / host_from_connect_args -- host extraction from socket address tuples and Unix socket paths
• _partition_allowed -- splitting allow-lists into plain hosts vs. CIDR networks (small, mixed, and large inputs)
• normalize_allowed_hosts -- resolving and normalizing host allow-lists, with and without a resolution cache
• disable_socket / enable_socket -- the core socket-patching cycle

CI workflow (.github/workflows/codspeed.yml)

• Runs on push to main, on pull requests, and on workflow_dispatch (for CodSpeed backtests)
• Uses simulation mode for deterministic, low-variance measurements
• Follows the existing workflow conventions (commit-hash pinning, uv, concurrency groups)

Other

• Added pytest-codspeed to the dev dependency group
• Added CodSpeed badge to README.md

Next steps

1. Merge this PR to establish the baseline measurements on main.
2. After the first run on main, CodSpeed will automatically report performance changes on future pull requests.
3. Visit the CodSpeed dashboard to explore flamegraphs and detailed metrics.

[codspeed-hq]

Congrats! CodSpeed is installed 🎉

🆕 12 new benchmarks were detected.

You will start to see performance impacts in the reports once the benchmarks are run from your default branch.

Detected benchmarks

• test_bench_disable_enable_socket: 77.6 µs
• test_bench_host_from_connect_args: 11.9 µs
• test_bench_is_ipaddress_invalid: 54.1 µs
• test_bench_host_from_address: 8.8 µs
• test_bench_host_from_connect_args_unix: 9.7 µs
• test_bench_partition_allowed_large: 721.9 µs
• test_bench_is_ipaddress_valid_ipv4: 37.5 µs
• test_bench_is_ipaddress_valid_ipv6: 58.6 µs
• test_bench_normalize_allowed_hosts_with_cache: 53.7 µs
• test_bench_partition_allowed_mixed: 80.7 µs
• test_bench_normalize_allowed_hosts_ips: 74 µs
• test_bench_partition_allowed_hosts_only: 12.3 µs

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