Reconstructs a signed Avalanche transaction from a custodian's offline secp256k1 signature. CDAO looks up the
stored immutable unsigned transaction by unsignedTxHash, verifies that the signer is authorized for that
transaction, attaches the signature, and returns signedTxSerialized ready for /submit.

This endpoint is shared by validator staking, delegation, C-to-P export, and C-to-P import flows. The signature
must be recoverable secp256k1 in r, s, and recoveryId form. recoveryId must be 0 or 1; do not send
Ethereum v values such as 27/28 or EIP-155-adjusted values. s must be canonical low-S.

This MVP contract supports one signer for threshold-1 UTXOs. For C-to-P export, the signer is the C-Chain source
address. For P-Chain stake, delegate, and import transactions, the signer is the P-Chain wallet. Multi-owner and
multi-signature P-Chain UTXOs are intentionally out of scope for this simple ETH-like flow.