add scenario 4 to single integrator example

[jpfairbanks]

Simple PR showing transitive tracking via consensus.

[Copilot]

Pull request overview

Adds a fourth scenario to the single-integrator target tracking documentation to demonstrate transitive tracking (an agent indirectly tracking a target coordinate via consensus with another agent).

Changes:

• Add “Transitive tracking” section with a new DSL-defined scenario (scenario 4).
• Generate and embed a new animation output (scenario4.gif) in the docs.
• Minor formatting adjustment to the existing Scenario 3 image line.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.60%. Comparing base (c379f01) to head (21f8026).

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #64   +/-   ##
=======================================
  Coverage   73.60%   73.60%           
=======================================
  Files          36       36           
  Lines        2550     2550           
=======================================
  Hits         1877     1877           
  Misses        673      673           

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