#1969: signing/verifying a message and explorer parser typo

[Maxnflaxl]

Summary

Adds message signing & verification (#1969): a wallet can sign an arbitrary
message with one of its own addresses to prove ownership, and anyone can verify
that signature against the address. Exposed on both the wallet CLI and the
wallet API (v7.4). Also fixes an explorer parser typo.

Wallet CLI

command	description
sign_message --message "<msg>" [--address <addr|*>]	Sign <msg> with one of your own addresses (--address defaults to *, the wallet's default address). Reads the signing key from the wallet, so it needs the wallet DB — but no node. Prints Signature: (hex) and Address: (the SBBS WalletID) to hand to the verifier.
verify_message --address <WalletID> --message "<msg>" --signature <hex>	Verify a signature against an address. Needs neither the wallet DB nor a node — it's pure signature math, so it runs fully offline. Prints Good signature from … / Invalid signature from … and sets the exit code.

Wallet API (v7.4)

• sign_message — reworked to sign with an own address: param address
  (preferred) instead of raw key material. The old key_material param is still
  accepted for backward compatibility (deprecated); if neither is given, the
  default address is used. Returns { "signature": "<hex>" }.
• verify_message (new) — params address (WalletID), message,
  signature; returns { "is_valid": <bool> }.
• verify_signature — unchanged; verifies against a raw public_key,
  returns { "result": <bool> }.

Protocol

CLI and API produce identical signatures, so a signature made by one verifies with the other.

• Digest: hv = H("beam.signed.message" | pubkey | len | message) — H is Beam's SHA-256 Hash::Processor, pubkey is the signer's 32-byte x-only WalletID key, and len is a width-independent varint. The fixed-width tag and key plus the explicit length make the encoding unambiguous.
• Key: the address's SBBS keypair (get_SbbsPeerID) — the key behind the WalletID — so a valid signature proves control of that address.
• Signature: Beam Schnorr (ECC::Signature) over hv, with a hedged nonce (secret key + message + fresh randomness).
• Verify: address.m_Pk.CheckSignature(hv, sig) — pure crypto; needs neither the wallet DB nor a node.

Binding the public key into the digest ties each signature to exactly one address — as in Bitcoin's address-recovery check and Monero's key-prefixed challenge — which prevents related-key signature malleability. Messages carry no replay/context binding, so anything using these signatures for authentication should embed a fresh challenge or context string in the message.

The legacy key_material form of sign_message and its counterpart verify_signature keep the original key-independent digest for backward compatibility.

Explorer fix

Corrected the AMM/DEX pool table header Rate 2:2 → Rate 2:1 in the rich
parser and rebuilt Parser.wasm.

Docs

Wallet API v7.4 reference: https://github.com/BeamMW/beam/wiki/Beam-wallet-protocol-API-v7.4