feat(extract): emit WHM+WOT+WOC for ADT terrain tiles

Final piece of the open-format emit pipeline:
  --emit-terrain  foo.adt → foo.whm + foo.wot + foo.woc

With this, --emit-open now produces a fully open-format zone
alongside every Blizzard MPQ extraction:
  BLP  → PNG       (textures)
  DBC  → JSON      (data tables)
  M2   → WOM       (models, with skin merge)
  WMO  → WOB       (buildings, with group merge)
  ADT  → WHM/WOT   (terrain heights + metadata)
       → WOC       (collision mesh derived from heights)

Originals stay on disk and indexed by manifest.json so private
servers continue to load proprietary formats; wowee runtime/editor
read the open formats directly. One extraction now feeds both
audiences with no separate conversion pass.

Implementation:
- Inline WHM+WOT writer in open_format_emitter.cpp (mirrors the
  editor's WoweeTerrain::exportOpen but without the PNG-preview /
  normal-map deps so the extractor stays editor-independent).
- Tile coords (x,y) parsed from <map>_<x>_<y>.adt filename.
- Collision mesh derived via WoweeCollisionBuilder::fromTerrain
  (terrain triangles only — WMO collision overlays would need
  asset manager and aren't worth the extractor complexity).

tools/asset_extract/extractor.hpp

@@ -36,6 +36,7 @@ public:
         bool emitJsonDbc = false;      // DBC → JSON side-files
         bool emitWom = false;          // M2 (+skin) → WOM side-files
         bool emitWob = false;          // WMO (+groups) → WOB side-files
+        bool emitTerrain = false;      // ADT → WHM + WOT + WOC side-files
     };
 
     struct Stats {