Fix PNG decoder: 16-bit channels, grayscale+alpha (colorType 4), and sub-byte formats

[Copilot]

• Explore repository and identify critical issues
• Fix PNG unfilterAndConvert bugs in src/formats/png_base.ts:
  • Fix 16-bit per-channel images: use x * bytesPerPixel stride and channelSize offset instead of hardcoded x*4, x*3, x
  • Add colorType 4 (grayscale+alpha) support (was throwing "Unsupported PNG color type: 4")
  • Fix sub-byte formats: scanlineLength now uses ceil(width * bitsPerPixel / 8) and pixel values are unpacked from packed bytes
  • Make sub-byte branch explicit: if (bitDepth < 8 && (colorType === 0 || colorType === 3)) to avoid silently mishandling future indexed-color support
• Add 3 new tests in test/formats/png.test.ts covering all three fixed cases
• Run precommit checks - all 552 tests pass
• Update CHANGELOG with fix descriptions

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[Copilot]

Pull request overview

This PR fixes critical bugs in the PNG decoder that prevented correct decoding of 16-bit per-channel images, grayscale+alpha (colorType 4) images, and sub-byte grayscale formats (bitDepth 1/2/4).

Changes:

• Fixed pixel stride calculation for 16-bit images to use actual channel size instead of hardcoded 8-bit offsets
• Added support for PNG colorType 4 (grayscale+alpha) which was previously unsupported
• Corrected scanline length calculation and pixel unpacking for sub-byte grayscale formats (bitDepth 1, 2, 4)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/formats/png_base.ts	Core decoder fix: replaces hardcoded pixel strides with dynamic calculations based on bitDepth and colorType, adds grayscale+alpha handling, and implements proper sub-byte pixel unpacking
test/formats/png.test.ts	Adds three decode-only tests with minimal hand-crafted PNG binaries to verify each of the three bug fixes
CHANGELOG.md	Documents the three decoder fixes in the Unreleased section under Fixed

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