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_scout.go

@@ -13,8 +13,8 @@ func handleMsgMhfPostGuildScout(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgMhfPostGuildScout)
 
 	err := s.server.guildService.PostScout(s.charID, pkt.CharID, ScoutInviteStrings{
-		Title: s.server.i18n.guild.invite.title,
-		Body:  s.server.i18n.guild.invite.body,
+		Title: s.I18n().guild.invite.title,
+		Body:  s.I18n().guild.invite.body,
 	})
 
 	if errors.Is(err, ErrAlreadyInvited) {
@@ -66,7 +66,7 @@ func handleMsgMhfCancelGuildScout(s *Session, p mhfpacket.MHFPacket) {
 func handleMsgMhfAnswerGuildScout(s *Session, p mhfpacket.MHFPacket) {
 	pkt := p.(*mhfpacket.MsgMhfAnswerGuildScout)
 
-	i := s.server.i18n.guild.invite
+	i := s.I18n().guild.invite
 	result, err := s.server.guildService.AnswerScout(s.charID, pkt.LeaderID, pkt.Answer, AnswerScoutStrings{
 		SuccessTitle:  i.success.title,
 		SuccessBody:   i.success.body,