feat(editor): add WSTC JSON round-trip (--export/--import-wstc-json)

Closes the editing loop on the hunter-stable-slot catalog: dump
a .wstc to JSON, hand-edit displayOrder / minLevelToUnlock /
copperCost / isPremium (e.g. lower the lvl 50 stable slot 4
unlock to lvl 45, mark slot 5 as premium for a donor server,
raise slot 3 cost from 2g to 5g for a tighter economy),
re-import to a byte-identical binary.

isPremium dual-encoded: bool OR int (machine-generated sidecars
work too). All other fields are scalar so no special handling
needed.

Verified byte-identical round-trip on all three presets
(std / cata / premium). CLI flag count 1074 -> 1076.

tools/editor/cli_arg_required.cpp

@@ -287,6 +287,7 @@ const char* const kArgRequired[] = {
     "--export-whld-json", "--import-whld-json",
     "--gen-stc", "--gen-stc-cata", "--gen-stc-premium",
     "--info-wstc", "--validate-wstc",
+    "--export-wstc-json", "--import-wstc-json",
     "--gen-weather-temperate", "--gen-weather-arctic",
     "--gen-weather-desert", "--gen-weather-stormy",
     "--gen-zone-atmosphere",