Phase 0: Cargo workspace + perplex-core crate (types, margin, eip712, api)

[ozpool]

Acceptance:

• Root Cargo.toml workspace defines all v1 crate members with workspace-level shared deps (tokio, axum, redis, sqlx, alloy 0.8, rust_decimal, proptest).
• perplex-core crate compiles cleanly with types.rs, margin.rs (mirrors Solidity math), eip712.rs (Order + SessionKey), api.rs (REST/WS shapes).
• perplex-core unit tests cover VWAP add / partial close / flip / full close / liquidation-price from docs/margin-math.md.
• 'cargo clippy --workspace --all-targets --all-features' passes with -D warnings.