rebuild: split core into separate repo; move wg-go integration to internal/ctr/corebind

- go.mod: require git.zkcoi.com/zkcoi/meshray/core, replace => ../Meshray
- internal/ctr/corebind: EnhancedBind + registry + adapter (wg-go integration isolated)
- cmd/mr-wg migrated from old core/cmd/meshray-core
- core/ removed; CHANGELOG updated; fix checkdb vet warning

pkg/contract/go.mod

@@ -0,0 +1,14 @@
+module git.zkcoi.com/zkcoi/meshray-contract
+
+go 1.26.0
+
+require (
+	go.uber.org/zap v1.27.0
+	golang.zx2c4.com/wireguard v0.0.0-20250521234502-f333402bd9cb
+)
+
+require (
+	go.uber.org/multierr v1.10.0 // indirect
+	golang.org/x/net v0.39.0 // indirect
+	golang.org/x/sys v0.32.0 // indirect
+)