Fix #131: add explicit zeroization in MPC sessions

[Woft257]

Changes

• Added security.ZeroBytes(...) for marshaled key/share buffers (keyData, keyBytes) in signing/keygen/reshare flows.
• Added security.ZeroEcdsaKeygenLocalPartySaveData(...) / security.ZeroEddsaKeygenLocalPartySaveData(...) to wipe key share fields after use.
• Added unit tests for zeroization helpers in pkg/security.

Files touched

• pkg/mpc/ecdsa_signing_session.go
• pkg/mpc/eddsa_signing_session.go
• pkg/mpc/ecdsa_keygen_session.go
• pkg/mpc/eddsa_keygen_session.go
• pkg/mpc/ecdsa_resharing_session.go
• pkg/mpc/eddsa_resharing_session.go
• pkg/security/zeroize.go
• pkg/security/zeroize_test.go

Notes

This improves deterministic cleanup; Go GC is treated as fallback.

[anhthii]

@vietddude please help review