K-Core early termination in PKC_original_serial

[adthoms]

Summary

• Add level-skipping optimization to PKC_original_serial in src/pkc.cpp: when no vertices exist at the current core level, jump directly to the next non-empty level by tracking the minimum degree above the current level during the vertex scan
• This avoids redundant iterations for sparse graphs with gaps in core numbers (e.g., K5 + isolated vertices skips levels 1-3 entirely)
• Add 4 new test sections verifying correctness: sparse graph with mixed core numbers, K5 + isolated vertices (large gap), complete graph (no gap), and empty graph

Test plan

• Verify PKC_SERIAL and BZ_SERIAL produce identical core numbers on mixed graphs
• Verify K5 + isolated vertices: cores 0 and 4 with levels 1-3 skipped
• Verify complete K5: max core number == 4, all 5 vertices in max k-core
• Verify empty graph: no crash, empty max k-core
• Existing random graph tests still pass (cross-validate all 4 solver modes)

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