Add DESIGN.md

[siburu]

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[Copilot]

Pull Request Overview

This PR adds a comprehensive design specification for the Optimism Enclave Light Client (ELC), detailing its architecture, data structures, key operations, security assumptions, and misbehaviour detection procedures.

• Introduces DESIGN.md capturing protocol overview, architecture, and verification workflows
• Defines core data structures (ClientState, ConsensusState, Header) and key functions
• Outlines security assumptions and provides Rust pseudocode for misbehaviour detection

Comments suppressed due to low confidence (3)

DESIGN.md:13

• Section 2 states there are only two verification operations (update_client and verify_membership), but later the spec includes verify_non_membership. Please update the overview to list all supported operations for consistency.

Functionally, OP ELC provides two types of verification operations, which together fulfill the stated objective:

DESIGN.md:58

• [nitpick] The sub-bullets under Preimage Oracle are not indented, which can reduce readability in rendered Markdown. Consider indenting the descriptions by two spaces under each main bullet.

- Online Preimage Oracle

DESIGN.md:144

• [nitpick] The Unicode approximate symbol (≒) may not render uniformly across all platforms. Consider replacing it with a plain-text alternative, e.g., "approximately equals", for clarity.

                - logic: `optimism_derivation::run` (≒ `kona_client::single::run`)