Implement 3rd harmonic using trigonometric identity

[akutuva21]

Replaced the hack/approximation in sp_fourier_synthesizer.bngl with an implementation of the 3rd harmonic based on the trigonometric identity: sin(3x) = 3*sin(x) - 4*sin^3(x). By taking the derivative, d(sin(3x))/dt = 3*cos(x)*w - 12*sin^2(x)*cos(x)*w, which corresponds in the BNGL functions to 3*ds1() - 12*s1()*s1()*ds1(). The ds3() function definition and comments were updated accordingly.

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PR created automatically by Jules for task 1254867357569153273 started by @akutuva21

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[akutuva21]

Reviewed in sequence. 3rd-harmonic derivative update is mathematically consistent with the stated identity and improves correctness.