Add Pauli spin blockade 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 simple 1200×700 numbered Pauli spin blockade sequence as a PNG.
• Adds the editable SVG source.
• Contrasts allowed singlet transport, (0,1) → S(1,1) → S(0,2), with the blocked triplet path that remains in T(1,1).

Verification

• Visually inspected the corrected rendered PNG against the established figure style.
• Confirmed isolated 1–2–3 badges and dashed step dividers in both rows.
• Confirmed opposite-spin singlet states with I_SD > 0 and parallel-spin triplet blockade with I_SD = 0.
• Confirmed PNG dimensions, matching SVG viewBox, charge-state labels, electron counts, and source-drain bias notation.

Limitations

• None.