feat(i18n): per-session i18n routing and localized scenarios

Phase C of #188 — the last phase of server-side multi-language support.

Adds Session.I18n(), a cached per-session i18n table resolver built via
getLangStringsFor(s.Lang()). The pointer is stable until SetLang
invalidates the cache, so hot-path handlers pay zero allocations on
repeated calls. All 51 s.server.i18n.* call sites across commands,
guild, guild scout, cafe, and cast-binary handlers now route through
s.I18n().*, so chat replies, guild invite mail templates, cafe reset
notices, and quest-timer broadcasts are served in the player's
preferred language instead of the server-wide default.

Scenario JSON gets the same plain-or-map LocalizedString treatment
that quests received in phase B: subheader Strings and inline entry
Text accept either a plain string (backwards compatible) or a
language-keyed object. CompileScenarioJSON takes the compiling
session's language, loadScenarioBinary passes s.Lang(), and
ParseScenarioBinary emits plain-string LocalizedStrings so existing
.bin files round-trip byte-for-byte through the JSON path.

World-wide broadcasts (Raviente siege announcements via
BroadcastRaviente) intentionally stay on the server default — they
have no single-session context to resolve against.

server/channelserver/handlers_guild.go

@@ -376,10 +376,10 @@ func handleMsgMhfEntryRookieGuild(s *Session, p mhfpacket.MHFPacket) {
 	// pkt.Unk==0: fresh rookie entering a rookie guild (return_type=1).
 	// pkt.Unk>=1: returning player entering a comeback/return guild (return_type=2).
 	returnType := uint8(1)
-	nameTemplate := s.server.i18n.guild.rookieGuildName
+	nameTemplate := s.I18n().guild.rookieGuildName
 	if pkt.Unk >= 1 {
 		returnType = 2
-		nameTemplate = s.server.i18n.guild.returnGuildName
+		nameTemplate = s.I18n().guild.returnGuildName
 	}
 
 	guildID, err := s.server.guildRepo.FindOrCreateReturnGuild(returnType, nameTemplate)