Let a requirement be optional, binding only a provider that already runs (hq issue 166, ADR 0330)

Making every soft dependency a provision made it hard: a mesh without influxdb could not assign
grafana, and one with influxdb in its catalogue would install it for grafana. A requirement may now
be written {"name": ..., "optional": true}. It resolves as a hard one when a provider is assigned or
reachable, and is otherwise absent: never refused, nothing installed for it. Its binding, credential
and only-when-bound files are left out, its ${bound:...|default:...} placeholders take their
defaults, the module check refuses one that could render empty, and plan and node list it unbound.

cmd/mesh-controller/unbound.go

@@ -0,0 +1,21 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/novox/mesh-controller/internal/catalogue"
+)
+
+// unboundLines say which optional requirements of a machine's modules nothing assigned answers
+// (novox/hq ADR 0330 §7): left unbound rather than refused, with nothing installed for them, so the
+// module runs without — and said, because a data source that never appears is otherwise a silence.
+// Each binds without anybody's act on the first composition after a provider is assigned.
+func unboundLines(plan catalogue.Resolution) []string {
+	lines := make([]string, 0, len(plan.Absent))
+	for _, a := range plan.Absent {
+		lines = append(lines, fmt.Sprintf(
+			"  unbound  %s, optional for %s — nothing assigned provides it; it binds once a provider is "+
+				"assigned (novox/hq ADR 0330)", a.Requirement, a.Consumer))
+	}
+	return lines
+}