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/merge_gate.go

@@ -135,6 +135,7 @@ type mergeComposed struct {
 	Problems  []string          `json:"problems,omitempty"`
 	Withheld  []string          `json:"withheld,omitempty"`
 	Unbound   []string          `json:"unbound,omitempty"`
+	Unplaced  []string          `json:"unplaced,omitempty"`
 	LeftOut   map[string]string `json:"left-out,omitempty"`
 	Resources []string          `json:"resources,omitempty"`
 }
@@ -372,6 +373,12 @@ func judgeChange(ctx context.Context, in mergeCheckInput) (mergeVerdict, error)
 			v.Failures = append(v.Failures, fmt.Sprintf("%s: the change leaves a credential bound elsewhere — %s",
 				gm.Described, u))
 		}
+		// A contribution its holder's template renders nothing for (novox/hq ADR 0255): the machine
+		// would be sent without it, so a change that adds one is refused, naming it.
+		for _, u := range newOnly(gm.Change.Unplaced, gm.Base.Unplaced) {
+			v.Failures = append(v.Failures, fmt.Sprintf("%s: the change leaves a contribution unplaced — %s",
+				gm.Described, u))
+		}
 		for module, why := range gm.Change.LeftOut {
 			if _, was := gm.Base.LeftOut[module]; !was {
 				v.Failures = append(v.Failures, fmt.Sprintf("%s: the change leaves %s out of its declaration — %s",
@@ -780,6 +787,7 @@ func composeEveryMachine(ctx context.Context, in mergeCheckInput, shelf map[stri
 			for _, u := range declared.unbound {
 				c.Unbound = append(c.Unbound, u.String())
 			}
+			c.Unplaced = declared.unplaced
 			sort.Strings(c.Withheld)
 			sort.Strings(c.Unbound)
 			out[m.Name] = c