refactor: extract gametime package, replace fmt.Printf with zap logging

Move time utilities (TimeAdjusted, TimeMidnight, TimeWeekStart, TimeWeekNext,
TimeGameAbsolute) from channelserver into common/gametime to break the
inappropriate dependency where signserver, entranceserver, and api imported
the 38K-line channelserver package just for time functions.

Replace all fmt.Printf debug logging in sys_session.go and handlers_object.go
with structured zap logging for consistent observability.

network/clientctx/clientcontext.go

@@ -1,4 +1,4 @@
 package clientctx
 
 // ClientContext holds contextual data required for packet encoding/decoding.
-type ClientContext struct{} // Unused
+type ClientContext struct{}