fix: BMP save/load uses BI_BITFIELDS for correct RGB565 color encoding

[Copilot]

save_image() wrote RGB565 BMP files with BI_RGB compression (0), causing standard viewers (Pillow, Windows, etc.) to assume RGB555 layout and corrupt the green channel for any mixed-color pixel.

Root cause

BITMAPINFOHEADER with 16bpp and BI_RGB is underspecified — readers default to 5-5-5. Correct RGB565 requires BI_BITFIELDS (compression=3) with explicit channel masks.

Changes

• lib/pygraphics/_bmp565.py — writer: Changed compression field from BI_RGB (0) → BI_BITFIELDS (3); appends three 4-byte masks (R=0xF800, G=0x07E0, B=0x001F) after the BITMAPINFOHEADER. Data offset updated 54 → 66 bytes.
• lib/pygraphics/_bmp565.py — reader: read_bmp565_header now reads and validates the compression field, accepting both BI_RGB (0) and BI_BITFIELDS (3) for backward compatibility.
• src/gfx_bmp565.c — writer/reader: Same fixes mirrored in the C implementation.

Before / After

# c1 = color565(0xFF, 0x8A, 0x3D) → 0xfc47
# Before fix (Pillow reads as RGB555):  (255,  16, 57)   ← green wrong
# After  fix (Pillow reads as RGB565): (255, 137, 57)   ← correct within 5/6-bit quantization

• Fixes #68