fix: correct Putnam-like Set_2/B6 integral identity

[sylvesterkaczmarek]

Summary

Fixes #7.

putnam_like/Set_2/B6 currently asks models to prove a false identity with cos x on the right-hand side. The corrected integrand is sin x.

This change:

• corrects the problem statement from cos x to sin x;
• repairs the grading scheme so the comparison function uses the sine integral consistently;
• fixes the differential-equation step to (f-g)'' + (f-g) = 0, matching the resulting trigonometric general solution;
• adds ERRATUM.md to preserve provenance for the released samples.

The existing samples/ directory is intentionally unchanged. Those files remain historical model outputs and grades produced against the original released cos x question.

Validation

I numerically cross-checked the three integrals with high-precision mpmath evaluation:

∫_0^∞ e^(-2x)/(1+x^2) dx  = 0.39902098859418384689
∫_0^∞ sin(x)/(2+x) dx     = 0.39902098859418384689
∫_0^∞ cos(x)/(2+x) dx     = 0.14454530303733242046

This confirms the sine version and rejects the published cosine identity. The corrected integration-by-parts derivation in the rubric yields g''(t) = 1/t - g(t), matching the corresponding equation for f.

No historical sample or grade files are modified.