Add Zeeman splitting illustration

[kiro-agent]

This pull request was created by @kiro-agent on behalf of @VanOschB 👻

Comment with /kiro fix to address specific feedback or /kiro all to address everything.
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Summary

• Adds a new 1200×760 Zeeman splitting illustration as a PNG.
• Adds the editable SVG source.
• Shows degenerate spin states at B = 0 and split levels at B > 0 with ΔE_Z = μ_B B.
• Matches the gold/blue scientific schematic style of the existing figures.

Verification

• Visually inspected the rendered PNG.
• Confirmed valid PNG dimensions and matching SVG viewBox.
• Confirmed both field regimes, spin labels, symmetric energy levels, and requested energy-separation formula.

Limitations

• None.