added diagrams for em1 chapter 01

[SG170]

Summary

This PR adds visual diagrams to EM1 Chapter 1 — Differential Calculus (/sem1/em1/ch1), which previously had only text content with placeholder comments.

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Changes made

• Added 4 SVG diagrams inside public/sem1/em1/
• Replaced all Diagram: placeholder comments in the chapter file with proper <img> tags

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Diagrams added

• mvt-graph.svg — Rolle’s theorem showing f(a)=f(b) with horizontal tangent at c, and Lagrange MVT showing chord slope equals derivative at c
• taylor-expansion.svg — Taylor series approximation of sin(x) showing T₁, T₃, T₅ polynomials converging to the actual curve
• concavity-inflexion.svg — S-curve showing concave up (f’’>0) and concave down (f’’<0) regions with inflexion point marked
• curvature-graph.svg — Curve approaching a horizontal asymptote and osculating circle showing radius of curvature R at point P

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[SG170]

@pushkarscripts, please review the pr.
thank you