test: expand error path tests for float8

[dndungu]

Summary

• Add NaN propagation tests through all arithmetic ops (Add, Sub, Mul, Div, Min, Max), negative NaN (0xFF), math functions, comparisons, slice operations, and Abs/Sign
• Add overflow clamping tests verifying out-of-range float32 values produce infinity (default mode) or errors (strict mode)
• Add underflow clamping tests for tiny float32 values clamping to zero
• Add comprehensive zero sign handling tests across all four arithmetic ops, division by zero, 0*Inf indeterminate forms, and zero equality

Test plan

• All new tests pass with go test -race ./...
• Pre-existing failures (TestArithmeticIdentities, TestAddBasic inf+(-inf)) are unrelated