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_test.go

@@ -5,27 +5,8 @@ import (
 )
 
 // TestClientContext_Exists verifies that the ClientContext type exists
-// and can be instantiated, even though it's currently unused.
+// and can be instantiated.
 func TestClientContext_Exists(t *testing.T) {
-	// This test documents that ClientContext is currently an empty struct
-	// and is marked as unused in the codebase.
-	var ctx ClientContext
-
-	// Verify it's a zero-size struct
-	_ = ctx
-
-	// Just verify we can create it
-	ctx2 := ClientContext{}
-	_ = ctx2
-}
-
-// TestClientContext_IsEmpty verifies that ClientContext has no fields
-func TestClientContext_IsEmpty(t *testing.T) {
-	// The struct should be empty as marked by the comment "// Unused"
-	// This test documents the current state of the struct
 	ctx := ClientContext{}
 	_ = ctx
-
-	// If fields are added in the future, this test will need to be updated
-	// Currently it's just a placeholder/documentation test
 }