feat(editor): add --add-tile to extend a zone with another ADT tile

--scaffold-zone creates a zone with one tile; some zones (continent
fragments, large dungeons) span multiple ADT tiles. This extends an
existing zone:

  wowee_editor --add-tile custom_zones/MyZone 29 30           # default baseHeight=100
  wowee_editor --add-tile custom_zones/MyZone 28 31 250.5     # custom baseHeight

What it does:
- Generates a fresh blank-flat WHM/WOT pair via the same factory
  --scaffold-zone uses, so output is consistent.
- Appends (tx, ty) to ZoneManifest::tiles. Save() rebuilds the
  files-block from tiles, so the new adt_TX_TY entry appears
  automatically in zone.json.

Safety:
- Tile coord must be in WoW grid [0, 64) per axis; rejects 99,99.
- Refuses if the tile is already in the manifest (catches typos).
- Refuses if the .whm/.wot files exist on disk but aren't in the
  manifest (catches manifest-out-of-sync drift from hand edits).
- Optional baseHeight allows seeding flat terrain at a non-default
  elevation.

Verified end-to-end: scaffolded 1-tile zone, added 2 more tiles
(one with custom height). Result: 3 tiles in manifest, 6 files on
disk, files-block has all 3 adt_TX_TY entries. Duplicate and
out-of-range cases both rejected with exit 1.

tools/editor/main.cpp

@@ -409,6 +409,8 @@ static void printUsage(const char* argv0) {
     std::printf("                         Convert a wowee JSON DBC back to binary DBC for private-server compat\n");
     std::printf("  --list-zones [--json]  List discovered custom zones and exit\n");
     std::printf("  --scaffold-zone <name> [tx ty]  Create a blank zone in custom_zones/<name>/ and exit\n");
+    std::printf("  --add-tile <zoneDir> <tx> <ty> [baseHeight]\n");
+    std::printf("                         Add a new ADT tile to an existing zone (extends the manifest's tiles list)\n");
     std::printf("  --add-creature <zoneDir> <name> <x> <y> <z> [displayId] [level]\n");
     std::printf("                         Append one creature spawn to <zoneDir>/creatures.json and exit\n");
     std::printf("  --add-object <zoneDir> <m2|wmo> <gamePath> <x> <y> <z> [scale]\n");
@@ -522,7 +524,8 @@ int main(int argc, char* argv[]) {
         "--unpack-wcp", "--pack-wcp",
         "--validate", "--validate-wom", "--validate-wob", "--validate-woc",
         "--validate-whm", "--validate-all", "--zone-summary",
-        "--scaffold-zone", "--add-creature", "--add-object", "--add-quest",
+        "--scaffold-zone", "--add-tile",
+        "--add-creature", "--add-object", "--add-quest",
         "--add-quest-objective", "--add-quest-reward-item", "--set-quest-reward",
         "--remove-creature", "--remove-object", "--remove-quest",
         "--copy-zone", "--rename-zone",
@@ -583,6 +586,11 @@ int main(int argc, char* argv[]) {
                 "--set-quest-reward requires <zoneDir> <questIdx> [--xp N] [--gold N] [--silver N] [--copper N]\n");
             return 1;
         }
+        if (std::strcmp(argv[i], "--add-tile") == 0 && i + 3 >= argc) {
+            std::fprintf(stderr,
+                "--add-tile requires <zoneDir> <tx> <ty>\n");
+            return 1;
+        }
         if (std::strcmp(argv[i], "--copy-zone") == 0 && i + 2 >= argc) {
             std::fprintf(stderr,
                 "--copy-zone requires <srcDir> <newName>\n");
@@ -3622,6 +3630,83 @@ int main(int argc, char* argv[]) {
                         slug.c_str(), slug.c_str());
             std::printf("  next step: run editor without args, then File → Open Zone\n");
             return 0;
+        } else if (std::strcmp(argv[i], "--add-tile") == 0 && i + 3 < argc) {
+            // Extend an existing zone with another ADT tile. Zones can
+            // span multiple tiles (e.g. a continent fragment), but
+            // --scaffold-zone only creates one. This adds another:
+            //   wowee_editor --add-tile custom_zones/MyZone 29 30
+            // Generates a fresh blank-flat WHM/WOT pair at the new tile
+            // and appends to the zone manifest's tiles list.
+            std::string zoneDir = argv[++i];
+            int tx, ty;
+            try {
+                tx = std::stoi(argv[++i]);
+                ty = std::stoi(argv[++i]);
+            } catch (...) {
+                std::fprintf(stderr, "add-tile: bad coordinates\n");
+                return 1;
+            }
+            float baseHeight = 100.0f;
+            if (i + 1 < argc && argv[i + 1][0] != '-') {
+                try { baseHeight = std::stof(argv[++i]); }
+                catch (...) {}
+            }
+            if (tx < 0 || tx >= 64 || ty < 0 || ty >= 64) {
+                std::fprintf(stderr, "add-tile: tile coord (%d, %d) out of WoW grid [0, 64)\n",
+                             tx, ty);
+                return 1;
+            }
+            namespace fs = std::filesystem;
+            std::string manifestPath = zoneDir + "/zone.json";
+            if (!fs::exists(manifestPath)) {
+                std::fprintf(stderr, "add-tile: %s has no zone.json — not a zone dir\n",
+                             zoneDir.c_str());
+                return 1;
+            }
+            wowee::editor::ZoneManifest zm;
+            if (!zm.load(manifestPath)) {
+                std::fprintf(stderr, "add-tile: failed to parse %s\n", manifestPath.c_str());
+                return 1;
+            }
+            // Reject duplicates so we don't silently overwrite an existing
+            // tile's heightmap when the user makes a typo.
+            for (const auto& [ex, ey] : zm.tiles) {
+                if (ex == tx && ey == ty) {
+                    std::fprintf(stderr,
+                        "add-tile: tile (%d, %d) already in manifest\n", tx, ty);
+                    return 1;
+                }
+            }
+            // Also bail if the file would clobber an existing one outside
+            // the manifest (e.g. user hand-created tiles without updating
+            // zone.json). Catches drift between disk and manifest.
+            std::string base = zoneDir + "/" + zm.mapName + "_" +
+                               std::to_string(tx) + "_" + std::to_string(ty);
+            if (fs::exists(base + ".whm") || fs::exists(base + ".wot")) {
+                std::fprintf(stderr,
+                    "add-tile: %s.{whm,wot} already exists on disk (manifest out of sync?)\n",
+                    base.c_str());
+                return 1;
+            }
+            // Generate the new heightmap. Reuses the same factory that
+            // --scaffold-zone uses, so the output is consistent.
+            auto terrain = wowee::editor::TerrainEditor::createBlankTerrain(
+                tx, ty, baseHeight, wowee::editor::Biome::Grassland);
+            wowee::editor::WoweeTerrain::exportOpen(terrain, base, tx, ty);
+            // Append + save manifest. ZoneManifest::save rebuilds the
+            // files block from the tiles list, so the new adt_tx_ty entry
+            // appears automatically in zone.json.
+            zm.tiles.push_back({tx, ty});
+            if (!zm.save(manifestPath)) {
+                std::fprintf(stderr, "add-tile: failed to save %s\n", manifestPath.c_str());
+                return 1;
+            }
+            std::printf("Added tile (%d, %d) to %s\n", tx, ty, zoneDir.c_str());
+            std::printf("  files     : %s.whm, %s.wot\n",
+                        (zm.mapName + "_" + std::to_string(tx) + "_" + std::to_string(ty)).c_str(),
+                        (zm.mapName + "_" + std::to_string(tx) + "_" + std::to_string(ty)).c_str());
+            std::printf("  tiles now : %zu total\n", zm.tiles.size());
+            return 0;
         } else if (std::strcmp(argv[i], "--copy-zone") == 0 && i + 2 < argc) {
             // Duplicate a zone — copy every file then rename slug-prefixed
             // ones (heightmap/terrain/collision sidecars carry the slug in