refactor(channelserver): consolidate tests into matching source test files

Move ~300 test functions from 21 catch-all files (handlers_core_test.go,
handlers_coverage*_test.go, *_coverage_test.go) into the *_test.go file
matching each handler's source file. This makes tests discoverable by
convention: tests for handlers_guild.go live in handlers_guild_test.go.

New files: handlers_guild_mission_test.go, sys_time_test.go.
No test logic changed — pure file reorganization.

server/channelserver/sys_time_test.go

@@ -0,0 +1,13 @@
+package channelserver
+
+import "testing"
+
+func TestTimeGameAbsolute(t *testing.T) {
+	result := TimeGameAbsolute()
+
+	// TimeGameAbsolute returns (adjustedUnix - 2160) % 5760
+	// Result should be in range [0, 5760)
+	if result >= 5760 {
+		t.Errorf("TimeGameAbsolute() = %d, should be < 5760", result)
+	}
+}