fix: chain AnySignatureError inner errors via #[source]
Add #[source] annotations to K256 and P256 variants in AnySignatureError
to properly chain wrapped curve-library errors through std::error::Error::source().
Consumers (miette, anyhow) can now traverse the full error context instead of
hitting a dead end at the high-level verification message.
Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>