[Fix] ARC module now supports ed25519 keys

[Fix] ARC module now supports ed25519 keys

- Remove hardcoded RSA-only restriction in do_sign()
- Replace manual RSA-specific key loading and signing in arc_sign_seal()
- Use native C dkim_sign() function with sign_type='arc-seal'
- Leverages existing C infrastructure that supports both RSA and ed25519
- Fixes 'DECODER routines::unsupported' error when loading ed25519 keys
- Algorithm detection (rsa-sha256 vs ed25519-sha256) now automatic
- Reduces arc_sign_seal() from ~100 lines to ~50 lines
- No FFI dependency, works with plain Lua installations

Resolves RSP-76