[integer] Polish and simplify the from_integral_scalar() method of BigInt

[forfudan]

This PR refactors BigInt.from_integral_scalar() to remove most dtype-specific branching by (1) computing the magnitude in an unsigned dtype of the same width and (2) extracting 32-bit limbs in a generic, compile-time unrolled loop. It also introduces a shared unsigned_counterpart() helper and updates the bigint enhancement plan document to describe the simplified approach.

Changes:

• Add decimo.utility.unsigned_counterpart[dtype]() to map signed integral dtypes to their equal-width unsigned equivalents.
• Rewrite BigInt.from_integral_scalar() to compute sign/magnitude generically and peel BITS_PER_WORD chunks in a single loop.
• Update docs/plans/bigint_enhancement.md with notes about simplifying from_integral_scalar under limb-width parameterization.

[Copilot]

Pull request overview

This PR refactors BigInt.from_integral_scalar() to remove most dtype-specific branching by (1) computing the magnitude in an unsigned dtype of the same width and (2) extracting 32-bit limbs in a generic, compile-time unrolled loop. It also introduces a shared unsigned_counterpart() helper and updates the bigint enhancement plan document to describe the simplified approach.

Changes:

• Add decimo.utility.unsigned_counterpart[dtype]() to map signed integral dtypes to their equal-width unsigned equivalents.
• Rewrite BigInt.from_integral_scalar() to compute sign/magnitude generically and peel BITS_PER_WORD chunks in a single loop.
• Update docs/plans/bigint_enhancement.md with notes about simplifying from_integral_scalar under limb-width parameterization.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
src/decimo/utility.mojo	Adds unsigned_counterpart() helper for mapping signed integral dtypes to unsigned counterparts.
src/decimo/bigint/bigint.mojo	Replaces dtype-by-dtype limb extraction with a generic magnitude + peel-loop implementation.
docs/plans/bigint_enhancement.md	Documents the refactor opportunity and considerations around limb-width parameterization.

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