Staked ETH that still behaves like a plain token

// The seed is minted to DEAD at construction. That fixes the opening
// rate at exactly 1.0000 and puts a floor under totalSupply, which is
// what makes the first-depositor inflation attack impossible.
constructor() payable {
    require(msg.value >= MIN_SEED, "seed below minimum");
    totalAssets = msg.value;
    _mint(DEAD, msg.value);   // 1:1 — sets the opening rate
}

// The rate is read out of two balances. Nothing writes it directly.
function convertToAssets(uint256 shares) public view returns (uint256) {
    return (shares * totalAssets) / totalSupply;
}

// Deposit ETH, mint kETH at the current rate. Quoted against
// pre-deposit balances, so you never mint against your own ETH.
function deposit() external payable returns (uint256 shares) {
    shares = convertToShares(msg.value);
    totalAssets += msg.value;
    _mint(msg.sender, shares);             // holders are not diluted
}

// Exit anytime: burn kETH for its current ETH value. Effects land
// before the transfer, and the whole call is nonReentrant.
function redeem(uint256 shares) external nonReentrant returns (uint256 assets) {
    require(balanceOf[msg.sender] >= shares, "insufficient balance");
    assets = convertToAssets(shares);
    _burn(msg.sender, shares);
    totalAssets -= assets;
    (bool ok, ) = msg.sender.call{value: assets}("");
    require(ok, "ETH transfer failed");
}

// Permissionless yield top-up — mints NO shares, so the rate rises
// for everyone. There is no privileged setter, and no owner
// withdrawal: deposited ETH cannot leave except through redeem().
function addYield() external payable {
    totalAssets += msg.value;
}