Variopinta 0.4.0

Variopinta 0.4.0

• Replace pipeline-global I/O and special-case masks with immutable Image and
  Mask targets. Each target selects an Array, Encoded, or Path carrier
  and fans its final raster out to named ReturnArray, ReturnTensor, Encode,
  and Write ports.
• Replace positional explicit calls and arity-collapsing tuples with keyword
  target bindings plus immutable PipelineResult/TargetResult values.
• Remove the Return output and ToTorch transform. ReturnTensor now owns
  the optional Torch adapter and the CHW terminal layout.
• Rename the public reference executor to Pipeline; .compile() returns
  CompiledPipeline with the same static target signature.
• Move semantic border fill to each Mask port, keep mask rasterization nearest
  and lossless, and make standalone I/O generic. PNG unchanged decoding
  preserves grayscale samples and palette indices from static 1/2/4/8-bit PNG.
• Make target writes atomic per file after all sources are acquired and every
  encoded result is prepared. Multiple destinations are not one transaction.