Improve quadratic and quartic Precision and Accuracy, add more tests

[Safari77]

1. Precision & Accuracy Discriminant Calculation: Replaced the naive bb - 4a*c with Kahan Summation and Veltkamp-Dekker Splitting. This effectively emulates higher precision (software FMA) to recover lost bits when b2≈4ac, fixing catastrophic cancellation errors.

Root Calculation: Switched to the Citardauq Strategy. The solver now calculates the larger root q first (using the sign of b) to avoid subtractive cancellation, then computes the second root as c/q.

2. Numerical Stability (Overflow/Underflow) Global Scaling: Implemented coefficient normalization. Inputs are scaled by 1/max(∣a∣,∣b∣,∣c∣) before calculation. This allows the solver to handle extreme values (e.g., 10300 or 10−300) without intermediate overflow or underflow, which the original version failed on.

Discriminant Tolerance: Added a check to treat tiny negative discriminants (caused by floating-point noise) as zero.

3. Robustness & Edge Cases Degenerate Cases: Added explicit handling for a=0 (linear fallback), c=0 (roots 0,−b/a), and b=0 (pure quadratic).

Safety: Added rigorous checks for NaN and Infinity inputs, returning Roots::No instead of undefined behavior.

4. Testing Comprehensive Suite: Ported the tests.jl test suite from https://github.com/goualard-f/QuadraticEquation.jl For quartic.rs, added more tests also.