Yield that never stops being an ordinary token

// The rate is derived purely from balances, so the vault can never owe
// more ETH than it holds. Yield fed in via addYield() lifts every rETH.

// ETH value of a given amount of rETH:
function convertToAssets(uint256 shares) public view returns (uint256) {
    return (shares * (totalAssets + 1)) / (totalSupply + 1e3);
}

// Deposit ETH, mint rETH at the current rate:
function deposit() external payable returns (uint256 shares) {
    shares = convertToShares(msg.value);   // assets and supply rise together
    totalAssets += msg.value;
    _mint(msg.sender, shares);             // depositors never dilute holders
}

// Withdraw anytime: burn rETH for its current ETH value:
function redeem(uint256 shares) external returns (uint256 assets) {
    assets = convertToAssets(shares);
    require(assets <= totalAssets, "insufficient liquidity");
    _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. This is where the protocol returns real staking /
// lending yield. No privileged setter can move the rate directly.
function addYield() external payable {
    totalAssets += msg.value;
}