Merge the /facts/<name> and /fact-names routes

Both fell to the unmerged pass-through, so one backend's answer was served
as if it were the estate's: Puppetboard's fact drilldown lost the other
backend's nodes and its facts overview lost that backend's fact names.

- Serve /facts/<name> and /facts/<name>/<value> through the /facts merge.
- Serve /fact-names as a deduped, re-sorted, re-paged union of name arrays.
- Gate provenance on the path: only /facts/<source-fact> may be injected.
- Keep the owned fact name out of /fact-names while the feature is on.
- Cache both alongside the merged /facts and /nodes record sets.
- Turn the two recorded e2e gaps into positive assertions.

source.go

@@ -39,6 +39,14 @@ func (si *sourceInjector) injects() bool {
 	return si != nil && si.inject
 }
 
+// disableInject rules the synthetic record out for a request shape the query
+// gate cannot see, leaving suppression on.
+func (si *sourceInjector) disableInject() {
+	if si != nil {
+		si.inject = false
+	}
+}
+
 // logSuppressed reports, once per request, that upstream records were dropped.
 func (si *sourceInjector) logSuppressed(l *log.Logger) {
 	if si == nil || si.suppressed == 0 || l == nil {