Add ERC: ERC-20 Pre-initialization Extension (Sentinel Storage)

[ariutokintumi]

This ERC proposes an optional ERC-20 extension that allows token contracts to implement a preInitializeAddress(address) function. This function lets users or dApps pay the high SSTORE gas cost of first-time balance initialization in advance, when gas is low, by writing a sentinel (magic) value to the balance slot.

The balance mapping is stored as bytes32 internally, and the sentinel is treated as 0 for all ERC-20 reads/writes. When the user later receives tokens, the sentinel is overwritten at the cheaper 5k gas rate instead of the full 20k.

We use bytes32 because writing zero to a slot does not save gas for the first real (nonzero) balance write. EVM only discounts overwrites of already-allocated slots with nonzero value.

This approach mirrors ERC-721A "gas timing" concept but in reverse: high gas now, low gas later, giving users more control over when they incur expensive storage writes.

Discussion thread: https://ethereum-magicians.org/t/erc-tbd-erc-20-pre-initialization-extension-sentinel-storage-gas-savings-for-first-time-token-receivers/24993

Repository including contracs, testing and tooling: https://github.com/ariutokintumi/ERC-20-Pre-initialization

[eip-review-bot]

File ERCS/erc-8003-erc20-preinit-sentinel-storage.md

Requires 1 more reviewers from @g11tech, @SamWilsn, @xinbenlv

[github-actions]

The commit ead73d5 (as a parent of e0b1d4a) contains errors.
Please inspect the Run Summary for details.