fix(math): stabilize decimal scaling for order amounts

[SebastianBoehler]

Summary

• add a small decimal math module for scaled-integer conversion
• route to_wei through the shared helper instead of stringifying a pre-truncated double
• tolerate tiny binary floating artifacts just below an integer scaled amount
• convert test_utils away from assert so Release builds actually execute the checks

Context

Fixes #15. A rounded trading value such as the representable double just below 3.03 could become 3029999 instead of 3030000 when converted to six-decimal order units.

Validation

• cmake -S . -B /tmp/polymarket-cpp-client-math-verify -DPOLYMARKET_CLIENT_BUILD_EXAMPLES=ON -DPOLYMARKET_CLIENT_BUILD_TESTS=ON -DPOLYMARKET_CLIENT_BUILD_BENCHMARKS=ON
• cmake --build /tmp/polymarket-cpp-client-math-verify -- -j4
• ctest --test-dir /tmp/polymarket-cpp-client-math-verify --output-on-failure
• git diff --check