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.

internal/catalogue/declaration.go

@@ -850,6 +850,11 @@ func (r Resolution) compose(with Rendering, owner map[string]string,
 		// An account depends on nothing the mesh computes.
 		accounts, rest := accountsFirst(resources)
 		resources = append(append(accounts, first...), rest...)
+		// **A file delivered only while its optional requirement is bound is left out while it is
+		// absent** (novox/hq ADR 0330 §5), and the field is dropped from the rest: the node-engine
+		// parses strictly and knows no such field.
+		absent := r.AbsentFor(m.Module)
+		resources = withoutAbsent(resources, absent)
 
 		// No container is given the mesh's names (novox/hq ADR 0148). It used to be: every
 		// container got the whole roster as `--add-host` entries at creation, and a name that
@@ -1013,7 +1018,7 @@ func (r Resolution) compose(with Rendering, owner map[string]string,
 			// **Substituted here, not on the machine.** A bound value is not secret — the mesh
 			// holds it in the clear — so there is nothing for the host to be the only witness of,
 			// and sending it already filled in means the host learns no new field.
-			if err := boundInto(copied, known, m.Module); err != nil {
+			if err := boundInto(copied, known, m.Module, absent); err != nil {
 				return nil, err
 			}
 			// And what the module could not have written: the machine it turned out to be