[rtwo2]

Summary

• Run CI once per pull request instead of also running it on every feature-branch push.
• Cancel stale CI runs when a newer commit is pushed to the same PR, and retain manual workflow_dispatch runs.
• Remove Validate Collector, whose validator self-test and collector build duplicate steps already covered by CI.
• Refresh CI actions to Node 24-compatible major versions.
• Document the new top_ipv6.txt output and clarify that the IPv4/IPv6 feeds are family-capable feeds, so dual-stack hostnames can appear in both.

Validation

• The latest full collection completed successfully on main before this PR was prepared.
• CI will validate the documentation/workflow cleanup on this PR.
• This change does not touch .github/workflows/collect.yml or collector source, so it should not trigger another full collection after merge.