Add test coverage for F2T function

[Copilot]

The F2T (FactorToThreshold) function lacked test coverage despite comprehensive testing patterns throughout the codebase.

Changes

• TestF2T: 14 test cases covering edge cases

  • Invalid inputs: zero, negative, NaN (all return NaN)
  • Infinity: +Inf returns 1.0, -Inf returns NaN
  • Common speedups: 1x (0.0), 2x (0.5), 3x (~0.667), 10x (0.9)
  • Slowdowns: values in (0,1) return negative thresholds
  • Extreme values: very large multipliers, near-zero inputs

• TestF2TEdgeCases: Boundary conditions and mathematical consistency

  • Exact behavior at timesFaster=1.0
  • Values just above/below 1.0
  • Formula validation: threshold = 1 - 1/timesFaster

// Examples of expected behavior
F2T(2.0)    // 0.5 (50% reduction, 2x speedup)
F2T(1.0)    // 0.0 (no speedup)
F2T(0.5)    // -1.0 (negative threshold, slowdown)
F2T(0.0)    // NaN (invalid)

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[TomTonic]

as expected