refactor(crypto): preserve Ed25519 wire format with dalek

[rldyourmnd]

Moves authentication signing from the unmaintained signing wrapper to ed25519-dalek 3.0 while preserving both deployed layouts: 64-byte seed || public key keypairs and signature || message signed payloads.

The compatibility boundary is executable: RFC 8032 vector 1 must produce the exact signature bytes, persisted keypairs with mismatched public halves are rejected, and modified signatures fail verification. The root server no longer declares sodiumoxide; the vendored common library still uses it only for the separate box_ and XSalsa20-Poly1305 secretbox protocol operations.

Validation:

• cargo test --locked --all-targets: 7 + 5 + 1 tests passed
• cargo test --locked --manifest-path libs/hbb_common/Cargo.toml crypto_sign: 2 passed
• cargo clippy --locked --all-targets -- -D warnings
• cargo fmt --all --check
• git diff --check