feat(editor): add WBGD JSON round-trip authoring workflow

Closes the WBGD open-format loop with --export-wbgd-json /
--import-wbgd-json, mirroring the JSON pairs added for
every other novel binary format. All 27 binary formats
added since WOL now have full JSON round-trip authoring.

Each battleground round-trips all scalar fields plus the
allianceStart and hordeStart vec3 + facing pairs (positions
become 3-element JSON arrays for natural hand-edit). The
objectiveKind enum emits dual int + name forms.

Verified byte-identical round-trip on the classic preset
(3 BGs spanning CTF / control nodes / resource race
objectives with full WMS map + WTKN markTokenId
cross-refs preserved through the JSON layer).

Adds 2 flags (637 documented total now).

tools/editor/cli_arg_required.cpp

@@ -97,6 +97,7 @@ const char* const kArgRequired[] = {
     "--export-wmou-json", "--import-wmou-json",
     "--gen-bg", "--gen-bg-classic", "--gen-bg-arena",
     "--info-wbgd", "--validate-wbgd",
+    "--export-wbgd-json", "--import-wbgd-json",
     "--gen-mail", "--gen-mail-holiday", "--gen-mail-auction",
     "--info-wmal", "--validate-wmal",
     "--gen-weather-temperate", "--gen-weather-arctic",