ci: preflight Linux smoke endpoint reachability

[Jesssullivan]

Summary

• add a Linux smoke endpoint DNS/TCP preflight before package install and daemon startup
• fail with a runner/endpoint classification when the smoke S3 endpoint cannot be resolved or reached from the selected runner

Why

The third TIN-1422 dispatch got past secrets, FUSE userspace, split package install, binary resolution, and version checks. It then timed out waiting for the daemon socket because tcfsd was blocked retrying S3 list calls to the shared smoke endpoint.

Failed run: https://github.com/Jesssullivan/tummycrypt/actions/runs/26072478492

Root cause from uploaded tcfsd.log:

• endpoint: http://seaweedfs-tcfs:8333
• hosted Ubuntu runner DNS failure: failed to lookup address information: Temporary failure in name resolution

This PR does not make a private endpoint public. It makes the workflow fail early and classify the required action: use a runner with private DNS access or a smoke environment with a public endpoint.

Validation

• ruby -e 'require "yaml"; YAML.load_file(".github/workflows/linux-postinstall-smoke.yml"); puts "yaml ok"'
• git diff --check

Refs TIN-1422.