feat(backend): add ETH/SOL public-key-to-address encoders

[AntonioVentilii]

Motivation

Prep for closing the OnRamper signing oracle (spec: docs/ai/spec-driven-development/specs/2026-06-22-fix-onramper-signing-oracle.md, draft PR #13164). To sign only the caller's own wallet addresses, the backend must derive them from the caller principal — which needs a way to turn a threshold public key into an Ethereum and a Solana address. The backend already does this for Bitcoin (btc_principal_to_p2wpkh_address) but had no ETH/SOL equivalent.

This is the first, additive PR in a 2-PR stack. It adds the pure pubkey→address encoders (no inter-canister calls), fully unit-tested in isolation. The follow-up PR wires the management-canister public-key reads and the endpoint rewrite, where integration tests prove end-to-end derivation parity against the signer.

Changes

• Add tiny-keccak (Keccak-256 — Ethereum's hash, distinct from NIST SHA-3) and bs58 as backend dependencies. Both were already in the lockfile; tiny-keccak is the crate used elsewhere in the dfinity Ethereum stack.
• src/backend/src/signer/service.rs:
  • eth_address_from_ecdsa_pubkey — EIP-55-checksummed Ethereum address from a SEC1 secp256k1 public key (keccak256 of the 64-byte uncompressed key, last 20 bytes, checksummed).
  • sol_address_from_ed25519_pubkey — Solana address (base58 of the 32-byte Ed25519 public key).
  • Helpers keccak256 and to_eip55_checksum.
• Both entry points are #[cfg_attr(not(test), expect(dead_code, ...))] until the stacked follow-up PR wires them into sign_onramper_widget_url.

Tests

New unit tests in signer::service::tests (all passing):

• ETH address matches the canonical known-answer vector for secp256k1 private key 1 (0x7E5F4552091A69125d5DfCb7b8C2659029395Bdf), proving both keccak derivation and EIP-55 checksum; compressed/uncompressed pubkey inputs agree; invalid pubkey rejected.
• SOL address of the all-zero pubkey is the known System-Program base58 string; base58 round-trips back to the pubkey; wrong-length pubkey rejected.

Local gates: cargo fmt, ./scripts/lint.rust.sh (wasm + native), targeted cargo test all pass.