refactor(editor): extract vertex weld into shared cli_weld utility

Three callers were each open-coding the same quantize-and-
bucket pass over vertex positions: --info-mesh-stats,
--info-wob-stats, and --bake-wom-collision. Move the
implementation to cli_weld.{hpp,cpp} as buildWeldMap() and
have each caller pass a flat positions array.

Identical std::map-based exact-equality keying preserved
(unbalanced-hash collisions remain absent). All three call
sites verified to produce byte-identical output:

  • info-mesh-stats firepit: 240→80 verts, 0 boundary
  • info-wob-stats cube:     8→8 verts, watertight YES
  • bake-wom-collision tent: 18→6 verts, 8-tri WOC

About 75 lines of duplication removed; future weld-using
commands now opt in by including one header.

tools/editor/cli_weld.hpp

@@ -0,0 +1,30 @@
+#pragma once
+
+#include <glm/glm.hpp>
+#include <cstddef>
+#include <cstdint>
+#include <vector>
+
+namespace wowee {
+namespace editor {
+namespace cli {
+
+// Vertex weld pass shared by --info-mesh-stats / --info-wob-stats /
+// --bake-wom-collision. Positions are quantized onto a 1/eps grid;
+// every vertex sharing a cell with a previously-seen vertex is
+// remapped to that vertex's index. Returns canon[v] giving the
+// canonical (lowest-index) representative of v's equivalence class
+// and writes the count of distinct cells to `uniqueOut`.
+//
+// Implementation uses std::map<tuple<int64,int64,int64>, uint32_t>
+// for exact equality on the quantized key — a hash-based key would
+// risk false-positive collisions that incorrectly merge distinct
+// corners (e.g. a unit cube's 8 corners all hashing to 2 buckets).
+std::vector<uint32_t> buildWeldMap(
+    const std::vector<glm::vec3>& positions,
+    float eps,
+    std::size_t& uniqueOut);
+
+} // namespace cli
+} // namespace editor
+} // namespace wowee