Restructure optimization content and add Fourier series section

Add §09-01 Entropy: notes.md (2000 lines), theory.ipynb (53 cells), exercises.ipynb (8 exercises)

Add §10 Numerical Methods: §03 exercises, §04 and §05 complete, chapter README

Add §10-01 Floating-Point Arithmetic and §10-02 Numerical Linear Algebra

Add §11-06 Random Graphs: notes, theory notebook, exercises

Fix §08-05 Stochastic Optimization notes.md: fences and formatting

Fix §08-04 Constrained Optimization notes.md: formatting

Fix §08-02 Gradient Descent notes.md: repair code fences and formatting

Fix §08-Optimization: correct syntax, repair code fences, complete TOC

Add §11-04 Spectral Graph Theory: notes, theory notebook, exercises

Add §11-03 Graph Algorithms: notes, theory, exercises

Add section 06: Optimization Landscape — critical points, sharpness, mode connectivity, saddle escape

Add §11-01 Graph Basics and §11-02 Graph Representations

Add sections 02-05 to Optimization chapter: Gradient Descent, Second-Order Methods, Constrained Optimization, Stochastic Optimization

Add §08-01 Convex Optimization: notes, theory notebook, exercises