Function upload_16bpp 

pub fn upload_16bpp(rect: VramRect, pixels: &[u16])

Upload raw 16bpp halfwords into a VRAM rect via GP0 0xA0 + word stream. Checks that pixels.len() * 2 == rect.w * rect.h – one halfword per pixel, and the FIFO ships 32-bit words containing two halfwords each. Odd pixel counts (which round up on hardware) panic because they’re rarely what the caller wanted.

For a 15bpp direct-color upload, each halfword is a Color555. For pre-packed 4bpp / 8bpp index data, each halfword packs 4 or 2 indices respectively.