spike: add ERC-7730 clear signing descriptor path - #132

Summary

• adds a minimal ERC-7730 descriptor model and context-bound normalizer for clear-signing review rows
• records the Deckard consumption path and trust/fallback rules in ADR 0005
• adds valid and invalid descriptor fixtures plus tests for descriptor-present rendering and invalid/mismatched fallback

Closes #65

Verification

• cargo fmt --all --check
• cargo test -p deckard-contract clear_signing
• just check
• cargo test --workspace

Notes

• No Cargo.toml or Cargo.lock changes.
• This commit is unsigned from Hermes; merge may require the user to amend/sign before merging until Hermes has its own signing key.