Bound a consumer's identity by the provision it requires (hq issue 263)

The one global 20-character bound made every consumer pay an object
store's key length, even for provisions that keep no name, and a single
overflow refused the provider's whole declaration. An offer now states
its own bound (identity: {max, in} or false); unsaid, a provider told its
consumers keeps 20 and one told nothing keeps none. module check judges
every identity on the longest machine name before merge, and a provider
leaves an overflowing consumer out of its grants and composes, with the
consumer named by push, plan and status (ADR 0225).

internal/catalogue/declaration.go

@@ -170,6 +170,10 @@ type Rendering struct {
 	// rather than resolved, because who consumes a node is a fact about the rest of the mesh and
 	// resolution answers questions about one machine.
 	Grants []Grant
+	// Withheld is every consumer left out of Grants because its identity overflows the provision's
+	// bound (novox/hq ADR 0225). Composed into nothing; carried so the machine's declaration can say
+	// whom it does not serve, and why, beside what it does.
+	Withheld []Overflow
 
 	// Ports is where this machine puts what each module needs reachable, by module and by the
 	// port the software itself uses (novox/hq ADR 0038).