chore(nodes): add a master+node end-to-end harness and CI job

Node sync had no test with two real panels: the single-panel tests either
call the node's controller in-process or hand-set the node-sync scope, so
823db059 shipped a regression no test could see (an admin-token node
dropped every client and enable flag its master pushed).

internal/nodee2e starts a master and a node as separate panel processes
(the DB is a process-wide global, so one process cannot be both) and walks
17 operations per enrollment scope (admin token, node-sync token): adopt,
create/edit on the master, small and bulk attach, client disable, detach,
client delete, inbound disable, traffic pull and reset, node-side delete
mirrored centrally (#6219), master-side delete, node down, node-lost
inbound re-created, create/edit while down, node disabled on the master,
and selected sync mode leaving unselected inbounds alone.

Against the build before 2ffc694a it fails 6 cells for an admin-token node
and 1 for a node-sync one; on main all 34 pass, on Windows and on Linux.
`make node-e2e` builds a stub-dist binary and runs it; ci.yml runs it as
its own job where a SKIP fails the build. Xray is not started, so cells
assert the node's stored state, not traffic through the core.

Makefile

@@ -58,6 +58,13 @@ test-go: dist-stub ## Go tests (shuffle, no cache)
 race: dist-stub ## Go tests with the race detector (needs a C compiler)
 	go test -race -shuffle=on -count=1 -timeout 25m $(GO_PKGS)
 
+.PHONY: node-e2e
+# Two real panel processes (master + node); test-go only runs nodee2e as a skip.
+NODE_E2E_BIN = $(CURDIR)/.cache/node-e2e/x-ui$(shell go env GOEXE)
+node-e2e: dist-stub ## Master+node sync end to end with two real panel processes
+	go build -o $(NODE_E2E_BIN) .
+	XUI_NODE_E2E_BINARY=$(NODE_E2E_BIN) go test -count=1 -timeout 20m -v ./internal/nodee2e/
+
 .PHONY: test-fe
 test-fe: ## Frontend tests (vitest)
 	cd $(FRONTEND) && npm test