feat(signing)!: deliver signature results by pointer

[piotr-roslaniec]

Breaking API change for signing callers.

common.SignatureData is a protobuf message and embeds a mutex. Delivering it by value over the signing end channel makes every receiver copy a lock, so go vet reports copylocks (keep-core already trips it). This switches to the API upstream BNB adopted in fbb0ef7, which this fork had deferred as unneeded API churn. The protobuf runtime upgrade (#19) is what made the by-value form wrong, and #37 already breaks the signing constructor, so making the change before the first tagged release avoids a second breaking release.

Changes

• signing.NewLocalParty / NewLocalPartyWithKDD take end chan<- *common.SignatureData.
• LocalParty stores its signature data by pointer, matching upstream.
• finalize sends proto.Clone(round.data), so the receiver owns a deep copy and shares nothing with the party. Upstream sends round.data itself; this is the only difference.
• Tests, the mixed-binary harness, and the README example are updated. historical_signer is unchanged because it builds against the pinned historical tss-lib.

Migration

endCh := make(chan *common.SignatureData, 1) // was chan common.SignatureData
// ...
sig := <-endCh // sig is *common.SignatureData

Verification

• go build ./..., go vet ./..., go test -run ^$ ./... (every test binary compiles): pass
• go test -race -shuffle=on ./ecdsa/signing: pass
• Mixed-binary harness: reject in round 3 with both culprits; accept and homogeneous runs reach round 8 on both sides
• Bidirectional historical transcript oracle: pass
• keep-core#4349 adapted and tested against this branch