fix: better edge case handling and test suite improvements

[mootz12]

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[Copilot]

Pull request overview

This PR refines the core fixed-point multiplication/division helpers to handle edge cases (sign handling, overflows, and division by zero) more robustly, and significantly expands the test coverage across integer widths and Soroban U256/I256 types.

Changes:

• Tightens mul_div_floor/mul_div_ceil implementations for u64, u128, i64, i128, U256, and I256 to use checked_* operations, correct sign-sensitive floor/ceil semantics, and safe widening fallbacks (u128/i128 and U256/I256).
• Greatly expands and restructures test suites to cover zero, negative, overflow, “phantom overflow”, and exact/rounding scenarios for both plain and Soroban-scaled fixed-point operations.
• Relaxes the soroban-sdk dependency constraint from 23.0.2 to the 23 major series in Cargo.toml.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/u64.rs	Refactors mul_div_floor/mul_div_ceil to use u128 as a checked widening fallback with u64::try_from, and replaces legacy fixed_* tests with direct mul_div_* tests plus updated fixed_* coverage.
src/u256.rs	Implements proper ceiling behavior in mul_div_ceil using remainder-based adjustment and adds comprehensive U256 tests including overflow, div-by-zero (panic) and helper for U256::MAX.
src/u128.rs	Updates div_ceil to use checked_div for safer division and introduces extensive tests for mul_div_*, scaled Soroban variants, and fixed_* operations over many edge cases.
src/i64.rs	Fixes sign-conditional logic in signed mul_div_floor/mul_div_ceil (true mathematical floor/ceil for negative rationals), uses i128 widening with try_from, and adds a rich matrix of signed test cases.
src/i256.rs	Corrects sign handling in I256 mul_div_floor/mul_div_ceil, adds helpers for 256-bit min/max, and significantly broadens tests for positive/negative, zero, and overflow scenarios and fixed_* methods.
src/i128.rs	Reimplements div_floor/div_ceil around sign-mismatch detection and checked_* ops, wires scaled Soroban variants through these helpers, and adds large, symmetric test suites for plain and Soroban fixed-point behavior; one minor comment in div_floor now mis-describes the subtraction (not the code).
Cargo.toml	Adjusts the soroban-sdk dependency to the 23 major version range to align with newer SDK releases.

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