fix: handle uints8 Long addition even in small field

[ivokub]

Description

For small fields our long (U32, U64) addition overflowed the native field. Modify the implementation to operate on bytes only to avoid overflows.

NB! Merge only after #1673 is merged.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How has this been tested?

• Test addition also with small fields.
• added more edge case tests

Checklist:

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• I did not modify files generated from templates
• golangci-lint does not output errors locally
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

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Note

Improves long-integer arithmetic and adds byte packing utilities.

• Refactors BinaryField.Add to accept variable inputs (including 0/1) and to switch between native add+partition and bytewise add-with-carry when maxBitlen exceeds the field size, preventing overflow on small fields
• Adds PackMSB, PackLSB, UnpackMSB, UnpackLSB for constructing/deconstructing T from bytes; adds zero() helper
• Documents bitwise ops and shifts; no functional change to And/Or/Xor/Not, Lrot, Rshift besides comments
• Tests: rewrite add circuit to slice-based and add cases for 0, 1, 2, and many inputs with WithSmallfieldCheck; add pack/unpack circuit tests

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[Copilot]

Pull request overview

This PR fixes a critical bug in the Add method for handling long integer (U32/U64) addition in circuits with small field sizes. When adding multiple long integers, the previous implementation could overflow the native field, leading to incorrect results. The fix adds conditional logic to detect when the result might overflow and switches to a bytewise addition approach with proper carry propagation.

Key changes:

• Added early-return optimizations for empty and single-element additions
• Split the addition logic into two branches based on field capacity
• Implemented bytewise addition with carry propagation for small fields

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