Megadev raster effects

Use raster effects when the design needs different display state within a frame. Mega CD still uses the Mega Drive VDP: extra CPUs, Word RAM and the ASIC do not add palettes, planes, sprite capacity or a second independent display. The Labs' scenery, audio, controls and split sizes are illustrative.

• Read Interrupt ownership and timing before installing a handler or changing VDP state during display.
• Read Water and split-screen for reflected tilemaps, line scroll, palette pairs, camera transforms and seam/sprite budgets.
• Read Validation for failure tests, regional builds, emulator checks and physical-hardware limits.

Inspect actual source in ewrogers/megadev:latest. The Lab's rt_* functions and raster_* assembly are application code, not Megadev raster APIs. Do not import SGDK interrupt, DMA-queue or sprite-engine calls.

Essential rules: