Unified signing trait and types for multi-chain transaction signers.

This crate defines the [Sign] trait that all chain-specific signer crates implement, plus shared types like [SignOutput] and [SignError].

Mirrors the role of kobe-core for derivation — this is the equivalent for signing.

Design

• Stateful signers — each Signer holds its private key internally.
• Send + Sync — signers are safe to share across threads and async runtimes.
• No address derivation — that's kobe's responsibility.
• Associated error type — matches kobe's Derive trait pattern.
• [SignExt] — blanket extension trait (like kobe's DeriveExt).