fix(release): tag immutable desktop candidates

[wesbillman]

Summary

Redesign the permanent Desktop release flow so unrelated merges to main cannot invalidate an already reviewed, green release candidate.

• Tag the immutable, API-confirmed release PR head instead of its later squash commit.
• Treat the merged PR—including an authorized owner/admin bypass—as publication authorization, while requiring trusted check evidence that was complete at merge time.
• Make tag creation idempotent and collision-safe: an existing tag succeeds only at the exact candidate SHA, and create races refetch before accepting equality.
• Replace ancestry-based previous-release discovery with a validated metadata ledger for side-history candidate tags.
• Compute the next release from the prior frozen base to the new frozen base, excluding only the prior release squash SHA so unrelated commits remain in the changelog.
• Preserve schema-1 production-tag migration and reject malformed metadata or equal/decreasing versions.
• Update operator documentation for the normal squash-merge workflow.

This is the reusable release process for 0.5.6 onward, not the retired one-shot 0.5.5 recovery path.

Invariants covered

• Candidate creation → unrelated main merge → authorized squash merge → immutable candidate tag.
• Trusted producer IDs and merge-time completion timestamps; DCO's bounded post-merge exception remains isolated.
• Missing/spoofed checks, tampered candidates, ambiguous PR associations, conflicting tags, and equal/decreasing versions fail closed.
• Same-SHA retries succeed; different-SHA collisions fail.
• Legacy schema-1 tag-on-main migration and schema-2 side-history accounting both preserve the correct next-release changelog.

Related issue

N/A — follows the Desktop release failures in #4788 and #4800 and the recovery revert in #4808.

Testing

At clean commit 6a91fbed8147a48cf174997de0c3e4cb2fb26474:

• scripts/test-desktop-release-candidate.sh
• scripts/test-release-ref-contract.sh

Both focused suites passed with HEAD unchanged. Princess Donut cleared the security/provenance surface, including the hostile merge-time timestamp cases. Mongo cleared the side-history ledger, migration, version-order, documentation, and contract-test surface.