feat(editor): add WVHC JSON round-trip (export/import sidecar)

Closes the JSON round-trip gap on the vehicle catalog format
shipped last batch. --export-wvhc-json emits all 10 scalar
fields plus a nested array of seats per entry, and dual int +
name forms for vehicleKind / movementKind / powerType so
hand-edits can use either representation. --import-wvhc-json
accepts either form, falling back to the int when both are
present, and reconstructs the seat array preserving seat
flags / attachments / control-spell cross-refs. Verified
byte-identical round-trip on all three preset emitters
(starter / siege / flying), including nested seat data. 702
documented CLI flags.

tools/editor/cli_arg_required.cpp

@@ -127,6 +127,7 @@ const char* const kArgRequired[] = {
     "--export-wgly-json", "--import-wgly-json",
     "--gen-vehicles", "--gen-vehicles-siege", "--gen-vehicles-flying",
     "--info-wvhc", "--validate-wvhc",
+    "--export-wvhc-json", "--import-wvhc-json",
     "--gen-weather-temperate", "--gen-weather-arctic",
     "--gen-weather-desert", "--gen-weather-stormy",
     "--gen-zone-atmosphere",