feat(hal): Add comprehensive ECDSA traits with curve markers (#22)

- Add complete ECDSA trait suite (key generation, signing, verification)
- Include security-first design with mandatory validation and
zeroization
- Add curve markers for P256, P384, P521, secp256k1, and Brainpool
curves
- Implement type-safe generic interface preventing cross-curve
contamination
- Add comprehensive error handling with detailed ErrorKind enum
- Include extensive documentation with security considerations and
examples.

Author: rusty1968