Validate CgBI chunks regardless of IHDR ordering

[Copilot]

CgBI compatibility validation previously ran only when CgBI preceded IHDR. Unsupported CgBI chunks placed after IHDR could bypass validation.

• Decoder validation

  • Validate CgBI constraints when a CgBI chunk is encountered after IHDR.
  • Preserve deferred validation for the normal CgBI-before-IHDR ordering.
  • Apply the same behavior to both decode and identify paths.

• Regression coverage

  • Add a malformed PNG fixture assembled with CgBI after a grayscale IHDR.
  • Assert Decode and Identify report the CgBI compatibility error.

case PngChunkType.ProprietaryApple:
    this.isCgbi = true;
    if (!Equals(this.header, default(PngHeader)))
    {
        ThrowIfInvalidCgbiContent(this.header);
    }

    break;

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[JimBobSquarePants]

No file changes. PR against the wrong branch. Absolute junk.

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