Add worked example to dim-si README

[dyreby]

Add a "Worked Example" section showing a complete kinetic energy calculation (KE = ½mv²).

Demonstrates:

• Typed function with Quantity<Mass>, Quantity<Velocity>, Quantity<Energy>
• Cross-dimension arithmetic (multiply, scale)
• Unit conversion (valueIn with joule/kilojoule)
• Compile-time error for dimension mismatches

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Stacked on #5 — merge that first.

Closes #8

[gdtroszak]

A few concerns with this PR as-is:

• Fluent vs free functions: The Quick Start showcases the q() fluent API, but this worked example uses only free functions (multiply, scale). This sends mixed signals about the preferred style and feels like a step backward from what the Quick Start already demonstrates.
• Import inconsistency: Types are imported from @isentropic/dim-isq directly, but the Type Annotations section (PR Add type annotation and compound unit docs to dim-si README #5) imports them from @isentropic/dim-si/* re-exports. Users will wonder which is correct.
• Redundancy: The example mostly re-demonstrates concepts already covered in the Quick Start (unit creation, arithmetic, conversions, compile-time errors) without adding much new.

Rather than a separate worked example, I think the better move is to expand the Quick Start to show more operations (multiplication, division, scaling) using the fluent API. I have pushed a commit to this branch that does that and removes the worked example section. The free functions get a one-line mention instead of a code block.

Please take a look and let me know what you think.