Implement the approved 4% GRID fee-on-transfer that the audit flagged as missing. Every transfer and transferFrom must take TAX_BPS = 400 from the sent amount; recipient gets amount minus tax.

Split tax: 50% City Rewards pool, 30% Resource Pot, 10% burn to address(0), 10% treasury 0x5b95A971B4583A5f011E9DA082acdD679b870D06. Emit TaxTaken. Add Foundry tests that fail if transfer/transferFrom are fee-free and that assert the four splits sum to the tax.

Do not change soulbound cities, buyCity pricing, levelUp, claimRewards, or GrantExecutor permissions. Independent adversarial review after the fix. Publish to GitHub.