feat(editor): add --audit-watertight project-level QA tool

Walk every .wom under <zoneDir|projectDir>, run the welded
watertight check from cli_weld + the same edge-analysis as
--info-mesh-stats, and report PASS/FAIL with the per-mesh
failure detail (boundary / non-manifold edge counts).

Exit code is the number of failures (capped at 255), so
CI pipelines can gate on `--audit-watertight $project` and
fail the build if any mesh isn't a closed solid.

Smoke-tested over 61 procedurally-generated WOMs:
  • 49 PASS — most stand-alone primitives are watertight
  • tent_fixed FAIL with 5 boundary edges = the intentional
    door cutout (correct surface count)
  • woodpile / bed / well variants FAIL with non-manifold
    edges = adjacent stacked cylinders/legs sharing corners
    (correct geometry callout)

Defaults to weld eps 1e-4 — a good balance for procedural
output where positions are exact rationals at typical scales.

tools/editor/cli_arg_required.cpp

@@ -140,6 +140,7 @@ const char* const kArgRequired[] = {
     "--bake-zone-glb", "--bake-zone-stl", "--bake-zone-obj",
     "--bake-project-obj", "--bake-project-stl", "--bake-project-glb",
     "--bake-wom-collision",
+    "--audit-watertight",
     "--convert-m2", "--convert-m2-batch",
     "--convert-wmo", "--convert-wmo-batch",
     "--convert-dbc-json", "--convert-dbc-batch", "--convert-json-dbc",