Default release delta to latest pair

[yvette-carlisle]

Summary

• make the release-delta module default to the canonical latest stable/prerelease pair even when that pair has zero curated signals
• keep older signal-bearing release windows available only through the selectors

Verification

• npm ci in site
• cargo make check-site-content
• npm run build
• npm run check
• git diff --check
• cargo make decodex-checks
• local preview curl confirms selected 0.130.0 -> 0.131.0-alpha.9 and hides 0.118.0 -> 0.119.0-alpha.13