Refactored Box getting libfuncs by CellExpression::add_with_const.

[orizi]

Summary

Added a new utility method add_with_const to CellExpression that creates a cell expression representing the sum of a cell reference and a constant value, with an optimization to return the cell directly when the value is zero. This method is then used to simplify and improve the implementation of boxed struct operations in the Sierra-to-CASM compiler.

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Type of change

Please check one:

• Bug fix (fixes incorrect behavior)
• New feature
• Performance improvement
• Documentation change with concrete technical impact
• Style, wording, formatting, or typo-only change

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Why is this change needed?

The code for handling boxed structs and local-to-box conversions contained duplicated logic for adding offsets to cell references. This PR extracts this common pattern into a reusable utility method, making the code more maintainable and less error-prone. Additionally, it refactors the struct boxed deconstruct implementation to handle offsets more consistently.

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What was the behavior or documentation before?

Before this change, offset calculations for boxed structs were implemented directly in each function that needed them, with duplicated logic for handling the special case of zero offsets.

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What is the behavior or documentation after?

After this change, a new utility method add_with_const handles offset calculations consistently, with proper optimization for zero offsets. The boxed struct deconstruction logic has been refactored into a separate helper function that uses this new utility.

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Additional context

This refactoring improves code maintainability while preserving the same functionality. The new boxed_members_cell_exprs helper function also provides better error handling and documentation for the boxed struct deconstruction process.

[orizi]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• Refactored EC-point point calculations. #9400
• Fixes the declared returned location of box-deconstruct. #9397
• Fixed cost of local_into_box cost. #9396
• Refactored Box getting libfuncs by CellExpression::add_with_const. #9393 👈 (View in Graphite)
• Merged SmallDiffHelper and build_128bit_diff fns. #9392
• Extended SmallDiffHelper documentation. #9391
• Added u128_bound function removing some minor calculations. #9390
• main

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[reviewable-StarkWare]

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[TomerStarkware]

@TomerStarkware reviewed 3 files and all commit messages, and made 1 comment.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @ilyalesokhin-starkware and @liorgold2).