feat: flatten last n layers.

[lingdie]

This pull request introduces a new feature to the flatten command, allowing users to flatten only the last N layers of an image instead of all layers. This adds flexibility for advanced image management and optimization. The implementation includes a new flag, updates to function signatures, and a new helper function for partial flattening.

Feature Addition: Partial Layer Flattening

• Added a --last-n-layers (-n) flag to the flatten command, enabling users to specify that only the last N layers of an image should be flattened. Includes validation to ensure the flag is non-negative. [1] [2]

Codebase Updates for Flag Support

• Updated all relevant function signatures (flatten, flattenIndex, flattenChild, flattenImage) to accept the lastN parameter and propagate it through the flattening logic. [1] [2] [3] [4] [5]

Core Logic Implementation

• Added the partialFlattenImage function, which merges only the last N layers into a single layer while preserving the earlier layers and original image history. This function handles layer extraction, merging, and history propagation, and ensures annotations are retained.

Input Validation

• Added validation in the command pre-run hook and in the partialFlattenImage function to ensure lastN is within a sensible range (1 to total layers). [1] [2]

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Let me know if you want a walkthrough of how the partial flattening works or have questions about the changes!

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