Vault: Share Token (ERC-20 + Permit)

What You Are Building

The vault IS its own ERC-20. The share token is the receipt for a pool stake; users hold it instead of the underlying. Section 4 will build the math that gives those shares value. This section builds the surface that lets users move them: transfer, transferFrom, approve, plus the internal primitives _mint, _burn, _transfer that later sections call from deposit and withdraw. We add EIP-2612 permit so users can approve via a signed message instead of a separate transaction.

We build ERC-20 from scratch, not by inheriting OpenZeppelin. Same choice Uniswap V2 made (UniswapV2ERC20 is hand-written) and Compound V2 made. The reason is pedagogical: every state mutation is visible in this file, no abstraction hides what _mint actually does.