Serve node-engine.compose-candidate from the node's shadow (H4)

ADR 0593 §7: a check's composition is the checker's, made from each
placed node's answer, since only a node can compose with its own sealed
values. The node composes the indexer's candidate in shadow on a copy of
its shadow's inputs, the candidate's modules laid over the indexer's
with their manifest verdicts, and answers per module composed (changed
or not), refused or held, and its own composition's verdict, within the
answer bound; it applies, writes and hands nothing. With input it also
answers the composition input, for the sandbox path.

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

cmd/mesh-host/nodeagent.go

@@ -623,6 +623,7 @@ func nodeAgent(ctx context.Context, opts options, signer ed25519.PublicKey, trus
 				session.NoteCandidates(ctx, delivery.WithValuesDiffer(answers))
 			})
 		go shadow.Keep(run, link.Backoff) // this run's alone: a switch begins a run, and its shadow, again
+		shadowNow.Store(shadow)           // what the verb compose-candidate composes a check's candidate with
 		// Its own composer, always (issue 1126, A7): a composition made elsewhere (the controller's, in the node's
 		// composition bucket) is never applied. While the seat's placement holds no key for this node it composes
 		// nothing and keeps running what it runs; a placement it cannot watch stops the agent, said, and it begins again.
@@ -640,6 +641,7 @@ func nodeAgent(ctx context.Context, opts options, signer ed25519.PublicKey, trus
 		cancel()
 		current.set(nil)
 		composerNow.Store(nil)
+		shadowNow.Store(nil)
 		if ctx.Err() != nil {
 			return
 		}
@@ -766,6 +768,10 @@ var machinesNow atomic.Pointer[selfcompose.Composer]
 // compositions or no run is going: what the verb compose asks a pass of (issue 774).
 var composerNow atomic.Pointer[selfcompose.Composer]
 
+// shadowNow is the shadow of the run now, which answers placement candidates and the verb compose-candidate; nil
+// between runs.
+var shadowNow atomic.Pointer[selfcompose.Shadow]
+
 // keptRetired says the kept declaration is read no more: every module the seat's placement puts on this node has a have of
 // its own (placedAllHave), so what runs is the agent's alone (ADR 0550 §2, design 47's L3 s9).
 var keptRetired atomic.Bool