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.

internal/catalogue/seat_data.go

@@ -49,17 +49,27 @@ type PlaceField struct {
 // OrderDefault is a piece's order when it gives none: the middle of 0–99.
 const OrderDefault = 50
 
-// capabilityName is what an `if-capability` may name: a capability as a node's profile reports it.
+// capabilityName is the shape of a capability's name, as a node's profile reports it.
 var capabilityName = regexp.MustCompile(`^[a-z0-9][a-z0-9-]*$`)
 
+// KnownCapabilities is every capability the node-engine detects (mesh-host internal/profile), the
+// names an `if-capability` may give (novox/hq ADR 0255). A name nothing detects would leave its
+// contribution out on every machine, silently, so it is refused. A detector added there is added here.
+var KnownCapabilities = []string{
+	"battery", "container-runtime", "firewall", "graphical-session", "overlay", "package-manager",
+	"power-meter", "privileged", "seat", "service-manager", "uplink-dhcpcd", "uplink-networkmanager",
+	"uplink-systemd-networkd", "virtualisation",
+}
+
 // renderFuncs are the functions a holder's template may call. `quote` writes a value as a JSON
-// string — a valid basic string in TOML and in most tools' grammars — so a contributed command
-// with quotes in it cannot break out of the holder's line.
-var renderFuncs = template.FuncMap{
-	"quote": func(v any) (string, error) {
-		b, err := json.Marshal(fmt.Sprint(v))
-		return string(b), err
-	},
+// string with DEL escaped as well — JSON leaves it bare, and TOML refuses it — which makes it a valid
+// basic string in TOML and in most tools' grammars, so a contributed command with quotes or control
+// characters in it cannot break out of the holder's line.
+var renderFuncs = template.FuncMap{"quote": quote}
+
+func quote(v any) (string, error) {
+	b, err := json.Marshal(fmt.Sprint(v))
+	return strings.ReplaceAll(string(b), "\x7f", `\u007f`), err
 }
 
 // shapedProblems is what is wrong with one contribution of a kind received as data.
@@ -197,7 +207,11 @@ type placedPiece struct {
 // shapedContributions is every module's data of one kind to one seat, narrowed by where and by the
 // machine's capabilities, ordered, and rendered through the holder's template — each piece under a
 // comment line naming its module.
-func shapedContributions(modules []Manifest, holder Manifest, s Seat, r Receivable, where map[string]string, caps map[string]bool) (string, error) {
+//
+// A piece the template renders to nothing — a `shows` the holder does not know yet — or fails on is
+// left out and answered as unplaced, naming its module: the other pieces and the rest of the machine's
+// declaration go on (novox/hq ADR 0255).
+func shapedContributions(modules []Manifest, holder Manifest, s Seat, r Receivable, where map[string]string, caps map[string]bool) (string, []string, error) {
 	var pieces []placedPiece
 	for _, m := range inModuleOrder(modules) {
 		for _, c := range m.allContributions() {
@@ -224,18 +238,24 @@ func shapedContributions(modules []Manifest, holder Manifest, s Seat, r Receivab
 		}
 	}
 	if len(pieces) == 0 {
-		return "", nil
+		return "", nil, nil
 	}
 	t, err := holderTemplate(holder, s, r.Kind)
 	if err != nil {
-		return "", err
+		return "", nil, err
 	}
 	sort.SliceStable(pieces, func(a, b int) bool { return pieces[a].order < pieces[b].order })
 	var b strings.Builder
+	var unplaced []string
 	for _, p := range pieces {
 		out, err := renderPiece(t, p.module, p.data)
+		if err == nil && strings.TrimSpace(out) == "" {
+			err = fmt.Errorf("%s's template renders nothing for it", holder.Module)
+		}
 		if err != nil {
-			return "", err
+			unplaced = append(unplaced, fmt.Sprintf("%s's %s:%s %s is not placed by %s: %v",
+				p.module, s.Name, r.Kind, describePiece(p.data), holder.Module, err))
+			continue
 		}
 		fmt.Fprintf(&b, "%s %s\n", r.Comment, p.module)
 		b.WriteString(out)
@@ -243,7 +263,19 @@ func shapedContributions(modules []Manifest, holder Manifest, s Seat, r Receivab
 			b.WriteString("\n")
 		}
 	}
-	return b.String(), nil
+	return b.String(), unplaced, nil
+}
+
+// describePiece is a piece in one line, its fields in order, for a person reading why it was left out.
+func describePiece(data map[string]any) string {
+	var parts []string
+	for _, k := range sortedKeys(data) {
+		if _, nested := data[k].(map[string]any); nested {
+			continue
+		}
+		parts = append(parts, fmt.Sprintf("%s=%v", k, data[k]))
+	}
+	return "(" + strings.Join(parts, " ") + ")"
 }
 
 // capable is whether a contribution applies on a machine with these capabilities: always, unless it