Leave out a block its holder cannot render, and keep if-capability to known names on offered kinds (hq ADR 0255)

A piece whose shows the holder's template does not know rendered as nothing and failed
the whole machine's declaration; it is now left out and named, for push, plan and the
merge gate. quote escapes DEL, which TOML refuses bare. An if-capability nothing
detects, or on a kind a holder depends on, would drop a piece silently, so both are
refused.

cmd/mesh-controller/plan.go

@@ -419,7 +419,7 @@ func declarationWith(ctx context.Context, open *stores, node string,
 	out := sendable{Resources: composed.Resources, Adoption: adoption,
 		Received: composed.Received, Mesh: with.Mesh, BusUsers: with.BusUsers,
 		LeftOut: sortedKeysOf(composed.LeftOut), leftOutWhy: composed.LeftOut, withheld: with.Withheld,
-		unbound: with.Unbound, foreseen: composed.Foreseen}
+		unbound: with.Unbound, foreseen: composed.Foreseen, unplaced: composed.Unplaced}
 	// And which build of each module it carries, for the send to record (novox/hq issue 259, ADR
 	// 0221). Read only on the send path: a question about what would be sent records nothing.
 	if choosing == Allocating {
@@ -528,6 +528,11 @@ func reportLeftOut(node string, declared sendable) {
 	for _, u := range declared.unbound {
 		fmt.Printf("%s: %s\n", node, u)
 	}
+	// And every contribution its holder could not render, which the machine is sent without (novox/hq
+	// ADR 0255).
+	for _, u := range declared.unplaced {
+		fmt.Printf("%s: %s\n", node, u)
+	}
 }
 
 // busCredentialIssued refuses an own secret called `broker` whose bus account nobody issued.