docs(ENG-314): Add eCLOB architecture sketch

[alnoki]

Changes

1. Add docs/architecture.md — a design sketch for the eCLOB (ephemeral central limit order book), positioning it between legacy CLOBs (fully-sorted onchain state, expensive per-quote rebalancing) and propAMMs (opaque per-maker accounts that fragment liquidity).
2. Document core data structures (Seat, ReferencePrice, Remaining, BookProfile, Level, MarketHeader, Registry) with field-level comments.
3. Describe the market account's contiguous memory layout (header + seat slab), DLL + free-list threading, and absolute SVM pointer handling.
4. Specify maker operations (OpenSeat, SetBookProfile, SetReferencePrice) and the authority/pointer validation scheme, including the zero-data-maker-account constraint.
5. Describe just-in-time order book reconstruction — the ephemeral heap-ordered book built per take — and the FLUSH_BIT protocol that lets makers defer per-level allowance refills onto taker flow.
6. Add clob, permissionless, pubkeys, and solana to the cspell dictionary.

[linear]

ENG-314