dual_decomposition: trim to the minimal-counterexample section

Remove the Introduction and Strategy sections and everything after the
separating-cycle definition (no-separating-triangle lemma, 5-connectivity
proposition, and the Step 2-6 stubs). Rename the section heading from
"Step 1: The minimal counterexample" to "The minimal counterexample", drop
the now-unused separating-cycle definition, and adjust the lead-in to mention
only the degree reduction. Remaining: reduction-to-triangulations lemma,
minimal-counterexample definition, |V|>=12 remark, and minimum-degree-5 lemma.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

papers/dual_decomposition_minimal_counterexamples/paper.fls

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