Fix MIS complexity for geometric graph variants to 2^sqrt(n)

KingsSubgraph, TriangularSubgraph, and UnitDiskGraph variants use
subexponential algorithms exploiting planar/geometric structure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: GiggleLiu

src/models/graph/maximum_independent_set.rs

@@ -162,11 +162,11 @@ fn is_independent_set_config<G: Graph>(graph: &G, config: &[usize]) -> bool {
 crate::declare_variants! {
     MaximumIndependentSet<SimpleGraph, i32>        => "1.1996^num_vertices",
     MaximumIndependentSet<SimpleGraph, One>         => "1.1996^num_vertices",
-    MaximumIndependentSet<KingsSubgraph, i32>      => "1.1996^num_vertices",
-    MaximumIndependentSet<KingsSubgraph, One>       => "1.1996^num_vertices",
-    MaximumIndependentSet<TriangularSubgraph, i32> => "1.1996^num_vertices",
-    MaximumIndependentSet<UnitDiskGraph, i32>      => "1.1996^num_vertices",
-    MaximumIndependentSet<UnitDiskGraph, One>       => "1.1996^num_vertices",
+    MaximumIndependentSet<KingsSubgraph, i32>      => "2^sqrt(num_vertices)",
+    MaximumIndependentSet<KingsSubgraph, One>       => "2^sqrt(num_vertices)",
+    MaximumIndependentSet<TriangularSubgraph, i32> => "2^sqrt(num_vertices)",
+    MaximumIndependentSet<UnitDiskGraph, i32>      => "2^sqrt(num_vertices)",
+    MaximumIndependentSet<UnitDiskGraph, One>       => "2^sqrt(num_vertices)",
 }
 
 /// Check if a set of vertices forms an independent set.