Test the merge against real openvoxdb backends

Every merge rule, AST gate and provenance decision was derived from reading
upstream source and proven only against fake backends, so nothing had ever run
against a real PuppetDB.

- Add an e2e suite behind the `e2e` build tag and a `make e2e` target
- Stand up two openvoxdb backends on their own PostgreSQL with testcontainers
- Load facts, reports and catalogs over the command API, waiting on processing
- Assert the union, freshness dedupe, summed aggregates, provenance gating,
  X-Backends, backend death and recovery, and the report paths
- Drive Puppetboard and node-lookup against pdbmux as real clients
- Record three known gaps as skips that fail once the gap closes

Makefile

@@ -5,7 +5,7 @@ GOFLAGS := -ldflags="-s -w -X main.version=$(VERSION)"
 OS      ?= $(shell go env GOOS)
 ARCH    ?= $(shell go env GOARCH)
 
-.PHONY: all build test lint fmt clean install patch minor major _tag
+.PHONY: all build test e2e lint fmt clean install patch minor major _tag
 
 all: build
 
@@ -15,6 +15,10 @@ build:
 test:
 	go test -v -race ./...
 
+# Needs a container runtime: two PostgreSQL, two openvoxdb and one Puppetboard.
+e2e:
+	TESTCONTAINERS_RYUK_DISABLED=true go test -tags e2e -race -count=1 -timeout=30m -v .
+
 lint:
 	golangci-lint run ./...