Implement MultiToken6909 in src/MultiToken6909.sol, an ERC-6909 multi-token with the EIP's exact interface and events: balanceOf(owner, id), allowance(owner, spender, id), isOperator(owner, spender), transfer, transferFrom, approve, setOperator, and supportsInterface for 0x0f632fb3 and ERC-165. Do not deploy.

Workspace: the job starts from an empty tree (no foundry.toml, forge-std or OpenZeppelin) and each worker may write only the listed src files and their test/.t.sol files, so no dependency can be added. Write everything inline (pragma ^0.8.24, default forge layout, no imports beyond these files); mocks and helper contracts live inside the test files, which declare the cheatcodes they use in an inline Vm interface at the HEVM address and assert with require.

Ids: create() returns the next id (starting at 1) and makes msg.sender its minter; mint(uint256 id, address to, uint256 amount) only by that id's minter and only for created ids; setMinter(id, newMinter) by the current minter (zero renounces for good); burn(uint256 id, uint256 amount) burns the caller's own balance; totalSupply(id) view.

Mint and burn emit Transfer with a zero sender or receiver; MinterSet(id, minter) on create and setMinter. transferFrom(sender, receiver, id, amount) precedence, per the EIP reference: caller == sender needs nothing; else an operator of sender uses no allowance; else allowance(sender, caller, id) must cover amount and is reduced unless it is type(uint256).max. Zero-amount and self transfers are allowed; transfers to address(0) revert (burn is the only way to reduce supply).

Tests: the precedence table (owner, operator with and without an allowance, spender with an allowance, max allowance left unchanged, insufficient allowance), operator revocation, allowances not leaking across ids, mint by a non-minter or for an uncreated id reverting, minter handover and renounce, events, supportsInterface, and a fuzzed invariant that each id's balances sum to its totalSupply.

Review focus: allowance versus operator precedence, minter takeover or id collisions, totalSupply overflow, and event correctness for indexers.