Add planarity test and PMFG (#208)

Co-authored-by: Stefan Krastanov <github.acc@krastanov.org>
Co-authored-by: Stefan Krastanov <stefan@krastanov.org>
Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

CHANGELOG.md

@@ -7,6 +7,7 @@
 - `maximum_clique`, `clique_number`, `maximal_independent_sets`, `maximum_independent_set`, `independence_number`
 - `regular_tree` generator
 - `kruskal_mst` now accepts weight vectors
+- `is_planar` planarity test and `planar_maximally_filtered_graph` (PMFG) algorithm
 - `count_connected_components` for efficiently counting connected components without materializing them
 - `connected_components!` is now exported and accepts an optional `search_queue` argument to reduce allocations
 - `is_connected` optimized to avoid allocating component vectors

docs/src/algorithms/spanningtrees.md

@@ -16,6 +16,8 @@ Pages = [
     "spanningtrees/boruvka.jl",
     "spanningtrees/kruskal.jl",
     "spanningtrees/prim.jl",
+    "planarity.jl",
+    "spanningtrees/planar_maximally_filtered_graph.jl",
 ]
 
 ```

src/Graphs.jl

@@ -449,8 +449,11 @@ export
     vertex_cover,
 
     # longestpaths
-    dag_longest_path
+    dag_longest_path,
 
+    # planarity
+    is_planar,
+    planar_maximally_filtered_graph
 """
     Graphs
 
@@ -573,6 +576,8 @@ include("vertexcover/random_vertex_cover.jl")
 include("Experimental/Experimental.jl")
 include("Parallel/Parallel.jl")
 include("Test/Test.jl")
+include("planarity.jl")
+include("spanningtrees/planar_maximally_filtered_graph.jl")
 
 using .LinAlg
 end # module