Isolate multi-platform release verification

[BrettKinny]

Summary

• remove each runner-local platform image after its exact-digest VERSION assertion
• prevent Docker classic image storage from reusing one manifest-list digest association across amd64 and arm64
• preserve remote digest identity, signing, and all later release gates
• add an ordering regression proving pull, verify, then removal remain inside the platform loop

RC7 root cause

Every runtime, lifecycle, evidence, and security gate passed. Release preparation pulled amd64 under the multi-arch digest, then Docker refused to overwrite that local digest association with arm64 and reported cannot overwrite digest.

Validation

• RC7 exact published digest passed native amd64 verification
• the same digest passed emulated arm64 verification after local reference removal
• all 17 executable test modules passed
• Actionlint 1.7.12 passed
• ShellCheck 0.9.0 passed
• workflow YAML and diff checks passed
• independent agent review found no blocking issue