Test coverage gaps: PPU, APU, controllers, mappers, browser, state serialization

[bfirsh]

Summary

Current test coverage is limited to:

• CPU instructions (cpu.spec.js)
• Basic NES initialization and ROM loading (nes.spec.js)
• Mapper 0 basics (mappers.spec.js)
• nestest and AccuracyCoin ROM harnesses

Missing test files for:

• PPU: register behavior, VRAM mirroring, sprite evaluation, scrolling, palette
• APU: channel behavior, frame counter, mixer, sweep unit
• Controllers: turbo button logic, clock(), strobe protocol
• ROM parser: trainer ROMs, truncated data, NES 2.0 headers
• Mappers 1-241: MMC1, MMC3, MMC5, etc. have zero unit tests
• State serialization: no round-trip toJSON()/fromJSON() tests
• Browser layer: Screen, Speakers, KeyboardController, GamepadController
• GameGenie: decode/encode round-trip, invalid input handling

The highest-value additions would be:

1. MMC3 IRQ counter unit tests (most complex and bug-prone mapper code)
2. PPU register behavior tests
3. State serialization round-trip tests
4. APU sweep unit tests