Phase B of #188. Quest JSON title/description/text_main/text_sub_a/
text_sub_b/success_cond/fail_cond/contractor now accept either a plain
string (existing behaviour) or a language-keyed object like
"title": { "jp": "...", "en": "...", "fr": "..." }
CompileQuestJSON takes the compiling session's language and resolves
each field through a fallback chain (requested -> plain -> jp -> en ->
any non-empty), so existing single-language quest JSONs keep working
byte-for-byte unchanged.
The quest cache is re-keyed on (questID, language) so compiled binaries
for different languages never leak between sessions on a multi-language
server. loadQuestBinary and loadQuestFile now pass s.Lang() into both
the compiler and the cache.
ParseQuestBinary emits plain-string LocalizedStrings, so the
binary -> JSON -> binary round-trip still produces identical output.
The new LocalizedString type lives in its own file and is reusable by
phase C (scenarios, mail templates, shop text). Shift-JIS encoding
still applies to the wire format, so localized values must use
characters representable in Shift-JIS — ASCII, kana, CJK — which is
documented on the type.
