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.
Add unit tests with race-detector coverage for QuestCache,
UserBinaryStore, and MinidataStore (18 tests covering hits, misses,
expiry, copy isolation, and concurrent access).
Document the lock acquisition order on the Server struct to prevent
future deadlocks: Server.Mutex → stagesLock → Stage → semaphoreLock.
