refactor: replace require strings with custom errors

[pplmaverick]

Summary

Replace 4 legacy require(string) statements with custom errors to align
with the existing convention used throughout src/interfaces/.

Changes

File	Change
src/sig/SignatureLib.sol	require → custom error
src/tx/Eip7702TransactionLib.sol	require → custom error
src/tx/Eip1559TransactionLib.sol	require → custom error
src/tx/TempoTransactionLib.sol	require → custom error

Motivation

• src/interfaces/ already uses custom errors exclusively
• Custom errors are more gas-efficient (no string stored in bytecode)
• No logic changes — behavior is identical

Testing

• forge build ✅
• forge fmt --check ✅
• forge test ✅ (3 existing tests, no regressions)

[onbjerg]

we can still use require and just pass the custom error as the 2nd arg

[pplmaverick]

Thanks for the suggestion! I considered using require(cond, CustomError()),
but this syntax requires solc ≥ 0.8.27 (0.8.26 only supports it with --via-ir).
Since the repo's minimum supported version is 0.8.20, this would break CI on
older compiler jobs.

Happy to switch if you'd like to bump the minimum version, otherwise I'll keep
the current if (...) revert pattern which is compatible across the full range.