Add NES PPU (Graphics): Nametables, Sprites, Palette, Rendering

Document NES PPU concepts in C.

Requirements:

• Define structs for PatternTable, Nametable, AttributeTable, and Palette
• Implement a function that prints an 8x8 tile from a pattern table as ASCII art
• Implement a function that prints a 32x30 nametable layout with tile indices
• Implement a function that shows palette indexing: print which palette is used for each 16x16 metatile
• Document sprite zero hit: print the conditions under which it occurs
• Print a summary of PPU memory map ($0000-$3FFF)
• Show a sample rendering pipeline: fetch pattern, fetch nametable, combine with attribute, output pixel
• Include a test that simulates rendering one scanline and prints the result