fix(mapper4): decode MMC3 register writes using mirrored address bits

[SergioChan]

Summary

• decode mapper 4 register writes using address & 0xE001 before dispatching cases
• keep behavior for canonical addresses unchanged while accepting valid mirrored addresses (for example $8002, $9FFE, $A002)
• add MMC3 unit tests covering masked bank-select and mirroring-register writes

Testing

• node --test test/mappers.spec.js
• note: npm test fails in this environment before tests run because prettier is not executable (sh: 1: prettier: Permission denied), so the targeted node test command was used

Related

Fixes #594