feat(evm): add Ed25519 signature-verify precompile at 0x403

[sfffaaa]

Summary

Adds a native EVM precompile for Ed25519 (RFC 8032) signature verification at address 0x403, flat 3,000 gas. Ed25519 is the scheme used by Substrate/Polkadot accounts, Solana and many off-chain systems; the EVM cannot verify it natively, which blocks cross-ecosystem attestations and Substrate-key-to-EVM identity proofs at reasonable cost.

Stacked on the P256VERIFY PR (#392) — this diff contains only the Ed25519 changes.

Interface

Address	0x0000000000000000000000000000000000000403 (peaq-specific, see below)
Input	128 bytes: message(32) ‖ publicKey(32) ‖ signature(64) — exact length, message is NOT pre-hashed
Output	32-byte 1 if valid; empty if invalid/malformed; never reverts
Gas	3,000 flat

Two deliberate decisions (reviewer attention)

Address is peaq-specific. Ed25519 has no adopted precompile-address standard: EIP-665 proposed 0x9 but was never activated (that slot is now Blake2F), and chains that added Ed25519 each picked their own (Celo 0xF3, Bittensor 0x402). 0x403 is the next free slot in peaq's non-Ethereum range (0x400 Sha3FIPS256, 0x402 ECRecoverPublicKey). Integrators must hardcode it for peaq only — needs to be explicit in docs.

Output convention matches P256VERIFY, not the upstream Frontier implementation this is vendored from. One check — out.length == 32 && out[31] == 1 — works for both signature precompiles. Vendoring is required because the upstream gas constants are compile-time and cannot be overridden from wiring; the upstream default (base 15 + 3/word) underprices an Ed25519 verify ~100×, a block-time DoS vector, hence the flat 3,000 matching ecrecover's pricing for comparable EC work.

Security notes

Standard RFC 8032 verification via ed25519-dalek 2.1.0: non-canonical s rejected (classic malleability blocked); small-order public keys accepted — the calling contract must treat the key as a binding it already trusts; replay protection and domain separation are the caller's responsibility. Wrong-length input (including trailing bytes) is rejected as invalid, never a revert.

Changes

New crate precompiles/ed25519 (no_std, adapted from Frontier), wired into peaq, peaq-dev and krest runtimes; callable by contracts and precompiles.

Testing

• Crate unit tests; all three runtimes compile at this commit.
• End-to-end against a live dev network built from this branch: valid signature → 0x…01; flipped message bit, truncated, oversized and empty inputs → empty output, no revert. Suite in peaq-bc-test branch feature/sig-precompile-tests.