TGA support - #51

Adds Truevision TGA (Targa) decoding and encoding. TGA is still common for game textures, sprites and screenshots, and in 3D/VFX pipelines.

Decoding

• Image types: color mapped, truecolor and greyscale, uncompressed or run length encoded (including packets that cross scan lines).
• Pixel formats:
  • color maps with 8/15/16/24/32 bit entries and an offset first index
  • truecolor with 15/16/24/32 bits
  • greyscale with 8 bits, or 16 bits including alpha
• Origins: all four (bottom/top, left/right).
• 16 bit images: the "attribute" bit is ignored, like image-rs does. Writers fill it inconsistently (always zero, inverted alpha, or unrelated data), so these images decode as opaque.
• Detection: TGA has no magic number, so ImageType.isTGA() / getType() === 'tga' recognize it by a consistent header or the TGA 2.0 footer. Over 2267 other image files (BMP, ICO, PSD, DDS, ...) it gave no false positives.
• Robustness: truncated data is rejected instead of being read past the end.

Encoding

Image.encodeTGA() writes run length encoded files: 24 bit for opaque images, 32 bit otherwise. They have a top left origin and the TGA 2.0 footer, so they are easy to recognize.

Verification

• Pillow and image-rs test files (53 TGAs):
  • 43 are pixel-identical to Pillow.
  • 6 are image-rs test files that Pillow can't read; we decode them. For the four origin variants, which store the same pixel data with different origin flags, the four decoded images are exact mirror images of each other.
  • 2 differ by design: 16 bit files where Pillow reads the attribute bit as inverted alpha.
  • 2 are broken files that both reject.
• Encoder: our TGAs round-trip losslessly and Pillow reads them pixel-identically.
• New tests: tests/tga.js builds TGAs byte by byte: bottom left origin with an image ID, a color map with a first index offset and top right origin, run length encoded greyscale + alpha with packets crossing scan lines, the 16 bit attribute bit, encoding round trips and truncated data.

Closes #21