compose-candidate reads the indexer's candidate by its group key, not the check's key

The checker asks by the composition key, <group>.<heads hash> (ADR 0593 §7), but the
indexer keys a candidate by its group alone (delivery.CandidatesKey). The node-engine
read the ask's key as the bucket key, so every check's composition was refused on
every node with "the indexer holds no candidate" (mesh-catalog #599, 2026-11-02).

It now parses the ask, reads CandidatesKey(group), and answers unready when the
group's candidate has moved to other heads. Both tests' fixtures had captured the
wrong key; they now key by group, with a slash in the group name.

Quick fix: this lookup. Thorough fix (a shared SDK reader that takes a composition
key and returns the candidate at those heads) not taken: one reader, one repository.

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

cmd/mesh-host/nodeagent_integration_test.go

@@ -575,13 +575,16 @@ func h8candidate(t *testing.T, b *itBed, fp, running string) {
 	heads, _ := delivery.HeadsHash([]delivery.Head{head})
 	pulls := []delivery.CandidatePull{{Head: head, Modules: []string{"web"},
 		Fps: map[string]delivery.CandidateModule{"web": {FP2: fpIt("c"), Manifest: digest}}}}
-	craw, err := delivery.EncodeCandidate(delivery.Candidate{Group: "novox/web#3", HeadsHash: heads, Pulls: pulls,
+	// The indexer keys the candidate by its group (a branch name, slash allowed); the checker asks by the composition key.
+	const group = "fix/3-web"
+	craw, err := delivery.EncodeCandidate(delivery.Candidate{Group: group, HeadsHash: heads, Pulls: pulls,
 		FPSet: delivery.FingerprintSet(pulls)})
 	if err != nil {
 		t.Fatal(err)
 	}
-	key, _ := delivery.CompositionKey("novox/web#3", heads)
-	if _, err := cands.Put(key, craw); err != nil {
+	ckey, _ := delivery.CandidatesKey(group)
+	key, _ := delivery.CompositionKey(group, heads)
+	if _, err := cands.Put(ckey, craw); err != nil {
 		t.Fatal(err)
 	}
 	before, _ := b.composed.Status()