Add nonlinear system demos post and static HTML demos

Author: famiuer

content/blog/nonlinear-system-study/index.md

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+---
+title: "A few nonlinear system demos"
+summary: "Quick note: I put together three small nonlinear system demos and shared them on my site."
+date: 2026-03-09T10:32:54+00:00
+authors:
+  - me
+tags:
+  - nonlinear-systems
+  - dynamics
+  - simulation
+---
+
+I put together a few lightweight interactive demos while studying nonlinear systems.
+
+If you want to play with them directly:
+
+- [Nonlinear pendulum (3D + phase cylinder)](/nonlinear/pendulum.html)
+- [Double-well potential](/nonlinear/double_well.html)
+- [Van der Pol oscillator (3D)](/nonlinear/vdp.html)
+
+Nothing fancy, just practical visual notes I can revisit later. I’ll keep adding more as I go.