Let a module-declared seat carry needs, so mail-notifier can need node-notifier (issue 802)

ADR 0220's dependency mechanism served compiled seats only and retired its one user with
node-resolver-config (commit f68521da). ADR 0378 S8 revives it for module-declared seats:
SeatDeclaration gains Needs, judged by the catalogue check (node scope only, a name that
resolves to a node seat somewhere in the catalogue, never itself), and DependsOn now takes
the catalogue so a claimed seat's needs are found wherever they are declared.

internal/catalogue/seats_declared_test.go

@@ -116,6 +116,53 @@ func TestTheMeshsOwnSeatsAreStillClaimable(t *testing.T) {
 	}
 }
 
+// Only a node seat may carry needs (novox/hq ADR 0378 §8): a per-mesh seat's holder is not on any
+// one node to depend on anything beside it there.
+func TestOnlyANodeSeatMayCarryNeeds(t *testing.T) {
+	m := Manifest{Module: "mail", DefinesSeats: []SeatDeclaration{{
+		Name: "mail-notifier", Scope: ScopeMesh, Needs: []string{"node-notifier"}}}}
+	got := strings.Join(declaredSeatProblems(m), "; ")
+	if !strings.Contains(got, "needing") || !strings.Contains(got, "node seat") {
+		t.Fatalf("a mesh-scope seat's needs were accepted: %q", got)
+	}
+}
+
+// A seat needing itself names nothing (novox/hq ADR 0378 §8).
+func TestASeatCannotNeedItself(t *testing.T) {
+	m := Manifest{Module: "mail", DefinesSeats: []SeatDeclaration{{
+		Name: "mail-notifier", Scope: ScopeNode, Needs: []string{"mail-notifier"}}}}
+	got := strings.Join(declaredSeatProblems(m), "; ")
+	if !strings.Contains(got, "needing itself") {
+		t.Fatalf("a seat needing itself was accepted: %q", got)
+	}
+}
+
+// A need naming nothing node-scope — undeclared, or declared at another scope — is the whole
+// catalogue's to say (novox/hq ADR 0378 §8).
+func TestANeedMustNameANodeSeatSomewhereInTheCatalogue(t *testing.T) {
+	mail := Manifest{Module: "mail", DefinesSeats: []SeatDeclaration{{
+		Name: "mail-notifier", Scope: ScopeNode, Needs: []string{"desk-beeper"}}}}
+
+	// Nobody declares desk-beeper at all.
+	got := problemsFor(t, Shelf{"mail": mail})
+	if !strings.Contains(got, "desk-beeper") || !strings.Contains(got, "no module declares") {
+		t.Fatalf("needing an undeclared seat was accepted: %q", got)
+	}
+
+	// Declared, but at mesh scope: still not a node seat to hold beside it.
+	notNode := Manifest{Module: "dunst", DefinesSeats: []SeatDeclaration{{Name: "desk-beeper", Scope: ScopeMesh}}}
+	got = problemsFor(t, Shelf{"mail": mail, "dunst": notNode})
+	if !strings.Contains(got, "desk-beeper") || !strings.Contains(got, "not a node seat") {
+		t.Fatalf("needing a mesh-scope seat was accepted: %q", got)
+	}
+
+	// Declared at node scope: accepted.
+	dunst := Manifest{Module: "dunst", DefinesSeats: []SeatDeclaration{{Name: "desk-beeper", Scope: ScopeNode}}}
+	if got := problemsFor(t, Shelf{"mail": mail, "dunst": dunst}); got != "" {
+		t.Fatalf("needing a node seat another module declares was refused: %q", got)
+	}
+}
+
 // A refusal that reorders itself between runs is a refusal nobody can diff.
 func TestTheProblemsAreStable(t *testing.T) {
 	shelf := Shelf{"telegram": telegram(), "shop": {Module: "shop", Uses: []string{"nope"}},