Add Nimbus release preflight gate

[ywkuno]

Summary

• Add a read-only Nimbus release preflight script for nimbus-v* tags.
• Check promoted release notes, installer fixture rows, local tag presence, and working-tree state before tagging.
• Document the preflight command in the release process.

Validation

• PowerShell parser check passed.
• git diff --check passed.
• Current nimbus-v0.1.0-alpha.1 preflight correctly reports BLOCKED while fixture rows are pending and final release notes are not promoted.
• Upstream-style tag 1.15.5 correctly reports BLOCKED.

Release Safety

This PR is read-only release hygiene. It does not create tags, push releases, install/uninstall anything, sign artifacts, publish symbols, publish WebRTC assets, or close issues.