Include fixed-group releases in website changelog

[SandroMaglione]

Summary

• Aggregate published and pending changelog entries across the Changesets fixed package group.
• Deduplicate shared entries and omit generated dependency-only release bullets.
• Fail site generation when the current package version is absent from the changelog page.

Changeset

• Added or updated for a library or package-metadata change
• Not required because this PR does not change a publishable package

Validation

• VITEST_MAX_WORKERS=1 pnpm check
• Automated type-performance measurement passed or was not required
• Automated runtime- and memory-performance measurement passed or was not required

pnpm docs:site renders the current v0.31.1 devtools release text, and the required local pnpm perf:runtime smoke test completed successfully.