[codex] Fix MMC3 sprite-fetch IRQ timing

[Kautenja]

Summary

• Add a mapper capability for PPU sprite-fetch address observations and enable it for MMC3.
• Emit mapper-visible dummy sprite fetches so MMC3 A12 scanline IRQ timing stays aligned even when no sprite pixels are drawn.
• Keep visible sprite rendering from emitting duplicate sprite-fetch address observations, which stabilizes SMB3 status-bar split baselines while sprites move.
• Document the fix, add native PPU/MMC3 regression coverage, and bump nes_py to 9.0.1.

Root Cause

The renderer only fetched sprite pattern data when it needed sprite pixels for composition. MMC3 titles also depend on the PPU sprite-fetch phase to produce A12 transitions for scanline IRQ timing. Without those mapper-visible fetches, split-screen effects such as the Super Mario Bros. 3 status bar could select the wrong CHR bank and render garbled HUD tiles.

After adding the sprite-fetch phase, visible sprite composition still performed mapper-visible CHR reads while drawing pixels. Those extra A12 observations depended on sprite positions, so SMB3's split IRQ could drift vertically during gameplay. Sprite drawing now reads pattern bytes without producing another mapper-visible sprite-fetch address event when the mapper already requested the dedicated sprite-fetch phase.

Validation

• build/nes-emu-debug/nes_emu_tests
• PYTHONPATH=/Users/christiankauten/Documents/Projects/gym-nes/nes-py /Users/christiankauten/Documents/Projects/gym-nes/gym-super-mario-bros/.venv/bin/python -m unittest discover
• PYTHONPATH=/Users/christiankauten/Documents/Projects/gym-nes/gym-super-mario-bros:/Users/christiankauten/Documents/Projects/gym-nes/nes-py /Users/christiankauten/Documents/Projects/gym-nes/gym-super-mario-bros/.venv/bin/python -m unittest discover
• Verified editable package metadata reports nes_py version 9.0.1.