chore(release): prepare beta-15

[jeremi]

Summary

• prepare Registry Stack beta-15 as version 0.12.1 without changing the preserved beta-14/v0.12.0 history
• add the release manifest and notes, align workspace, lockfiles, OpenAPI, product notes, current docs, archived docset, installers, starters, and fixtures
• refresh the Notary and Relay advisory baselines from exact Linux/amd64 candidate images and require the tagged images to reproduce the reviewed root filesystems

Release identity

• release ID: beta-15
• version: 0.12.1
• tag: v0.12.1
• frozen pre-prepare source: d6d2d16
• Crosswalk pin: 1d44ec735fdc8a7c719264b339574371e8330337

Verification

• read-only prepare plan: ready, 17 bounded changes and 10 passing checks, including remote tag absence
• 236 release-tool tests
• full documentation check, all archived docsets, and 524,007 built-link checks
• two clean pinned-builder Linux/amd64 builds matched byte for byte across four image inputs and six release binaries
• exact OCI labels passed for both candidate images
• both Grype gates passed with zero fixable, unreviewed, expired, invalid, or stale blocking findings
• Cargo metadata, formatting, deny policy, Debian image contract, gate inventory, and both product preflights passed

The immutable v0.12.0 tag is not reused. This PR does not create or push v0.12.1.

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