Merge main + fix coverage gaps for SteinerTree

Resolve merge conflicts with ExactCoverBy3Sets (#603), regenerate
reduction_graph.json, and add tests for edge_weights/set_weights/weights
methods and constructor validation panic paths to close coverage gaps.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: GiggleLiu

docs/paper/reductions.typ

@@ -278,6 +278,7 @@ pred create MaxCut --graph 0-1,1-2,2-0 -o maxcut.json
 pred create SteinerTree --graph 0-1,0-3,1-2,1-3,2-3,2-4,3-4 --edge-weights 2,5,2,1,5,6,1 --terminals 0,2,4 -o steiner.json
 pred create Factoring --target 15 --bits-m 4 --bits-n 4 -o factoring.json
 pred create Factoring --target 21 --bits-m 3 --bits-n 3 -o factoring2.json
+pred create X3C --universe 9 --sets "0,1,2;0,2,4;3,4,5;3,5,7;6,7,8;1,4,6;2,5,8" -o x3c.json
 ```
 
 Canonical examples are useful when you want a known-good instance from the paper/example database.

docs/src/cli.md

@@ -89,6 +89,22 @@
       }
     ]
   },
+  {
+    "name": "ExactCoverBy3Sets",
+    "description": "Determine if a collection of 3-element subsets contains an exact cover",
+    "fields": [
+      {
+        "name": "universe_size",
+        "type_name": "usize",
+        "description": "Size of universe X (must be divisible by 3)"
+      },
+      {
+        "name": "subsets",
+        "type_name": "Vec<[usize; 3]>",
+        "description": "Collection C of 3-element subsets of X"
+      }
+    ]
+  },
   {
     "name": "Factoring",
     "description": "Factor a composite integer into two factors",