feat(tbtc): validate migration destination reservation artifacts

[mswilkison]

Summary

• extend the covenant signer request contract with a concrete migration destination reservation artifact
• validate reservation script hash, migration extraData encoding, and canonical destination commitment hash inside keep-core
• add fixed-vector and mismatch coverage so later real signing can rely on a verified destination artifact instead of only an opaque hash

Why now

The new tBTC covenant migration destination reservation service is merged on the covenant project branch. The keep-core signer substrate now needs to consume that concrete artifact so later self_v1/qc_v1 signing slices can fail closed on destination mismatch before implementing real tx construction.

Verification

• go test ./pkg/covenantsigner
• go test ./config
• go test ./cmd