Spike: consume ERC-7730 descriptors for clear signing

[0xpantera]

Part of #46 (PRD-02: Clear-signing v2 + message-signing intents).

Why

#46 already calls out consuming ERC-7730 descriptors where available, but we should track the spec and Clear Signing docs as a first-class implementation spike so Deckard's message-signing UX can align with the emerging descriptor format instead of inventing an incompatible local schema.

References:

• ERC-7730 / EIP-7730: https://eips.ethereum.org/EIPS/eip-7730
• Clear Signing build docs: https://clearsigning.org/build/

Scope

• Read ERC-7730 and the Clear Signing build docs.
• Identify the minimum descriptor fields Deckard needs for EIP-712 typed-data clear signing.
• Decide where descriptor lookup/parsing lives: contract model, signerd policy layer, app rendering layer, or a dedicated resolver module.
• Define fallback behavior when no descriptor exists or descriptor validation fails: explicit blind-sign / undecodable typed-data warning, matching PRD-02: Clear-signing v2 + message-signing intents #46.
• Add fixtures for at least one representative descriptor and one malformed/unsupported descriptor.

Questions to answer

• Can Deckard consume descriptors directly, or do we need a normalized internal ClearSigningDescriptor representation?
• How should descriptors be trusted, versioned, cached, and invalidated?
• Which parts are safe to show as human-readable fields without over-trusting third-party metadata?
• How does descriptor consumption interact with domain binding (chainId, verifyingContract) and unknown-contract warnings?

Acceptance criteria

• Short design note or ADR describing Deckard's ERC-7730 consumption path.
• Parser/normalizer spike with fixtures, or a documented decision to defer implementation with concrete blockers.
• Clear UI behavior for descriptor-present, descriptor-missing, and descriptor-invalid cases.
• Tests covering descriptor-present rendering and descriptor-invalid fallback.

Non-goals

• Building or hosting an ERC-7730 registry.
• Treating descriptor metadata as a security oracle.
• Allowing auto-approval of message signatures; PRD-02: Clear-signing v2 + message-signing intents #46 keeps message signing human-approved in v1.