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

README.md

@@ -451,6 +451,35 @@ curl -s --get http://localhost:8080/pdb/query/v4/nodes \
 
 `make build` (static binary into `dist/`), `make test`, `make lint`. Requires Go 1.25+.
 
+## End-to-end tests
+
+`make e2e` runs the suite against **real** PuppetDB backends: two openvoxdb
+containers, each on its own PostgreSQL, loaded over the command API
+(`replace facts` v5, `store report` v8, `replace catalog` v9, `deactivate node`
+v3) and queried through `pdbmux`. Two real clients — Puppetboard and, when a
+binary is available, `node-lookup` — are pointed at `pdbmux` and asserted on. It
+needs a container runtime and takes a couple of minutes, so it sits behind the
+`e2e` build tag and never runs as part of `make test` or `go test ./...`.
+
+Commands are submitted with `secondsToWaitForCompletion`, so the harness waits
+on PuppetDB actually processing each one rather than sleeping, and the fixture
+load ends by polling `queue_depth` on `/status/v1/services` until both backends
+have drained.
+
+| Env var | Overrides |
+|---|---|
+| `PDBMUX_E2E_OPENVOXDB_IMAGE` | `ghcr.io/openvoxproject/openvoxdb:8.15.0` |
+| `PDBMUX_E2E_POSTGRES_IMAGE` | `docker.io/library/postgres:17-alpine` |
+| `PDBMUX_E2E_PUPPETBOARD_IMAGE` | `ghcr.io/voxpupuli/puppetboard:latest` |
+| `PDBMUX_E2E_TUNNEL_IMAGE` | `docker.io/library/alpine:3` |
+| `PDBMUX_E2E_NODE_LOOKUP` | path to a `node-lookup` binary (else `PATH`, else skipped) |
+
+Three tests skip rather than assert, each naming a known gap and failing if that
+gap closes: `/facts` aggregates are not summed, `/pdb/query/v4/facts/<name>` is
+served unmerged (so Puppetboard's single-fact drilldown silently loses the other
+backend's nodes), and `/fact-names` is likewise unmerged (so the facts overview
+lists only the first backend's fact names).
+
 ## Deployment
 
 Container image only — no OS package. Every `v*` tag builds and pushes the image