Align ERC-20 returns and complete ERC20TransferFacet fuzz unit tests

[lumoswiz]

Summary

Align ERC-20 module/facet behavior with ERC-20 returns and finish ERC20TransferFacet fuzz coverage per the shared ERC-20 tree.

Changes Made

• Updated ERC20Mod approve/transfer/transferFrom to return bool.
• Propagated return values through ERC20Harness.
• Added ERC20TransferFacet base test and scaffolds for facet transfer/transferFrom tests.
• Implemented ERC20TransferFacet fuzz tests covering all tree branches, including zero-amount paths.

Checklist

Before submitting this PR, please ensure:

• Code follows the Solidity feature ban - No inheritance, constructors, modifiers, public/private variables, external library functions, using for directives, or selfdestruct

• Code follows Design Principles - Readable, uses diamond storage, favors composition over inheritance

• Code matches the codebase style - Consistent formatting, documentation, and patterns (e.g. ERC20Facet.sol)

• Code is formatted with forge fmt

• Existing tests pass - Run tests to be sure existing tests pass.

• New tests are optional - If you don't provide tests for new functionality or changes then please create a new issue so this can be assigned to someone.

• All tests pass - Run forge test and ensure everything works

• Documentation updated - If applicable, update relevant documentation

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Name	Link
🔨 Latest commit	78df956

[github-actions]

Coverage Report

Metric	Coverage	Details
Lines	74%	1183/1605 lines
Functions	85%	295/349 functions
Branches	54%	122/224 branches

Last updated: Tue, 06 Jan 2026 20:42:05 GMT for commit 78df956

[github-actions]

Gas Report

No gas usage changes detected between main and test/erc20-transfer-facet.

All functions maintain the same gas costs. ✅

Last updated: Tue, 06 Jan 2026 20:42:29 GMT for commit 78df956