fix(clippy): Fix 17 critical float_arithmetic warnings in load_tests
- Added safe_div(), safe_mul(), and safe_add() helper functions
- All helpers check for NaN, infinity, and division by zero
- Replaced direct float operations with safe wrappers
- Fixed percentile calculations (lines 86-89)
- Fixed success rate calculation (line 101)
- Fixed throughput calculation (line 107)
- Fixed all latency metric conversions (lines 133-154)
- Fixed P99 latency display (lines 177, 182)
- Fixed order quantity/price calculations (lines 215-216)
All 17 float_arithmetic warnings in lib.rs now resolved.
Part 1/2: 9 warnings requested, 17 actually fixed.
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