Clarify Pixel-Image relationships as 1-1

[stellarsunset]

Move the Pixel sub-classes into their associated image types to clarify all concrete image types are generally expected to have their own semantically meaningful pixel handle.

[codecov]

Codecov Report

❌ Patch coverage is 52.94118% with 32 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.62%. Comparing base (c357a9c) to head (6b6dde0).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ub/stellarsunset/tiff/baseline/GrayscaleImage.java	25.00%	8 Missing and 1 partial ⚠️
...o/github/stellarsunset/tiff/baseline/RgbImage.java	0.00%	5 Missing ⚠️
...github/stellarsunset/tiff/extension/ByteImage.java	60.00%	4 Missing ⚠️
.../github/stellarsunset/tiff/extension/IntImage.java	60.00%	4 Missing ⚠️
...ithub/stellarsunset/tiff/extension/ShortImage.java	60.00%	4 Missing ⚠️
...thub/stellarsunset/tiff/baseline/BiLevelImage.java	57.14%	2 Missing and 1 partial ⚠️
...stellarsunset/tiff/baseline/PaletteColorImage.java	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main      #38   +/-   ##
=========================================
  Coverage     73.62%   73.62%           
+ Complexity      233      232    -1     
=========================================
  Files            72       71    -1     
  Lines          1638     1638           
  Branches        188      188           
=========================================
  Hits           1206     1206           
  Misses          342      342           
  Partials         90       90           

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