Fix off-by-one bounds check in layout layer facade

[arpitjain099]

The two bounds checks in newLayerOrFacadeFrom use > where they should use >=, so when layerIndex equals the slice length the guard doesn't fire and we fall straight through to configFile.RootFS.DiffIDs[layerIndex] / manifestFile.Layers[layerIndex]. That's an index out of range panic instead of the error the guard was there to return.

It's reachable through the layout facade: newImageFacadeFrom ranges over original.Layers() and hands each index to newLayerOrFacadeFrom, which then indexes into the config's diff_ids. Config and manifest get parsed independently and nothing cross-checks the counts, so an OCI layout image whose manifest lists more layer descriptors than the config has diff_ids (or the other way round) loads fine and then panics when it's used as a base or previous image. Only affects the facade path, so data-less/sparse layers.

Changing > to >= gets you the intended "failed to find layer for index N" error. Valid images are unaffected since a valid index never equals the array length.

Added layout/v1_facade_test.go covering both directions of the mismatch plus an in-bounds case. It's an in-package test since newLayerOrFacadeFrom isn't exported. Panics before the change, passes after, and go test ./layout/... is green.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 34.78%. Comparing base (c36b3eb) to head (ac7d98e).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #308      +/-   ##
==========================================
+ Coverage   34.66%   34.78%   +0.12%     
==========================================
  Files          39       39              
  Lines        3359     3359              
==========================================
+ Hits         1164     1168       +4     
+ Misses       2000     1998       -2     
+ Partials      195      193       -2     

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[jabrown85]

Thanks for this!