Diagramming & STEM Tools section has only 5 entries

[yakew7]

Problem

The ## Diagramming & STEM Tools section has only 5 entries (Desmos, diagrams.net, Excalidraw, GeoGebra, Wolfram Alpha). It's one of the thinner top-level sections in the list.

What to do

Add 1-2 more vetted entries to the existing ## Diagramming & STEM Tools section, in correct alphabetical position among the current entries.

Acceptance criteria

• New entry/entries follow the existing bullet format inside the current <details open> block
• Entries meet the Quality Standards (real, maintained, pricing noted)
• Table of Contents / badge counts regenerated with node scripts/update-counts.mjs
• node scripts/check-list-format.mjs passes

Scope: small, roughly 20 minutes.

[yakew7]

Added PhET Simulations and Tinkercad to Diagramming & STEM Tools (5 -> 7 entries), in alphabetical order. Both verified live. Counts regenerated via node scripts/update-counts.mjs, node scripts/check-list-format.mjs passes.