[integer] Rewrite and fix type conversion from all integral scalar types

[forfudan]

Fix broken conversion from Int128 and add proper support for all integral types (Int128, Int256, UInt128, UInt256, platform-sized Int/UInt).

The old code cast everything through UInt64, silently truncating wide integers. The Int.MIN special-case was also fragile.

Now from_integral_scalar dispatches per dtype at comptime:

• Small unsigned types → single word
• UInt64 → 2-word split
• UInt128/UInt256 → loop with bit ops
• Signed ≤64-bit → unsigned subtraction for magnitude
• Int128/Int256 → division-based chunk extraction (right-shift on signed wide ints was broken)
• Platform Int/UInt → size_of dispatch

Removed standalone from_uint64 and from_uint128 (folded into from_integral_scalar). Updated sqrt call sites accordingly.

Added tests for Int.MIN/MAX edge cases, Int128/Int256 MIN/MAX, UInt128.MAX, UInt256, platform-sized types, and zero across all types.

[Copilot]

Pull request overview

Fixes BigInt construction from 128-bit scalar integer types (Int128/UInt128) by updating the generic scalar conversion path and extending the conversion test suite accordingly.

Changes:

• Extend test_from_integral_scalar to cover UInt128 and Int128 (including MIN/MAX).
• Rework BigInt.from_integral_scalar word-extraction logic to handle wider scalar types beyond 64-bit.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/bigint/test_bigint_conversion.mojo	Adds coverage for UInt128/Int128 scalar-to-BigInt conversions (small/large, MIN/MAX).
src/decimo/bigint/bigint.mojo	Updates scalar conversion implementation to extract 32-bit limbs from wider scalar types.

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

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