Stop the source-fact drilldown fanning out per pinned value

The <value> path segment is client-supplied and reached a full unfiltered
/facts fan-out, keyed per value, so every distinct value was a fresh
whole-estate query and a fresh cache entry.

- validate <value> against the configured backend names, answering [] with
  no fan-out when it names none
- key the drilldown's fetch on the fact name alone and apply <value> to the
  shared record set, so all values share one entry and one fan-out
- report every configured backend on the no-fan-out empty response, which is
  complete rather than partial

cache_test.go

@@ -96,6 +96,18 @@ func (cb *countingBackend) hitCount(path string) int {
 	return cb.hits[path]
 }
 
+// totalHits counts requests across every path, so a test can assert a route
+// reached no backend at all.
+func (cb *countingBackend) totalHits() int {
+	cb.mu.Lock()
+	defer cb.mu.Unlock()
+	n := 0
+	for _, c := range cb.hits {
+		n += c
+	}
+	return n
+}
+
 func (cb *countingBackend) setFail(v bool) {
 	cb.mu.Lock()
 	defer cb.mu.Unlock()