Add SecureBytes for secure memory wiping of seed data

[r2ckstardev]

Summary

Adds SecureBytes wrapper that uses ctypes.memset to zero sensitive seed bytes and share data from memory after use.

Why

After Codex32 shares reconstruct the master seed, raw bytes stay in Python memory until GC runs. On an air-gapped SeedSigner, that leaves private key material exposed in RAM.

Usage

from secure_bytes import SecureBytes

with SecureBytes(seed_bytes) as sb:
    root = bip32.HDKey.from_seed(sb.get())
# buffer zeroed here

Context manager for automatic cleanup, or call .wipe() manually. No changes to existing code - additive only.