An assignment says how far an endpoint reaches, and three things read it

novox/hq ADR 0138. Reachability was settled three times over: the filter read a
listen's source with expose able to override it; the proxy composed a public name
and an internal name for every route it was given, because it could; and the
certificate authority followed from which names existed. Each was defensible and
the combination was unstated, so "this endpoint must not be public" could not be
written and was enforced by nothing — while a public certificate for that name was
obtained anyway. Measured on the control node: an identity provider holding a
90-day public certificate and a 24-hour internal one, neither asked for.

`reach` is one value per endpoint, per node — machine, internal, public or both —
and the filter's source and the composed names both follow it. The authority needs
no work: the proxy already asks the public authority for a route's own name and its
internal authority for the internal one, so controlling the names controls the
authority.

Joined by the port, which a route already names: 35 of the catalogue's 36 route
entries name a port the same module declares a listen on, and the one that does not
is a path-level refusal — a rule about a name rather than an endpoint, left alone.

Nothing said composes both names and follows the manifest's `from`, so every mesh
already running is unchanged until an assignment speaks. A port that says both
reach and expose is refused: they say the same thing in different words, and the
filter would follow one while the names followed the other.

internal/catalogue/declaration.go

@@ -897,6 +897,24 @@ func (r Resolution) Rules(with Rendering) ([]Rule, error) {
 		if err != nil {
 			return nil, err
 		}
+		// And how far each endpoint reaches, which says the same thing to the filter and more
+		// besides (novox/hq ADR 0138). Folded in here rather than beside: the filter has one
+		// question — from where — and a reach answers it, so giving it two inputs would let them
+		// disagree. Reaches refuses a port that both name, so this cannot silently prefer one.
+		reaches, err := Reaches(m, with.Settings[m.Module])
+		if err != nil {
+			return nil, err
+		}
+		for port, reach := range reaches {
+			source, ok := FilterSource(reach)
+			if !ok {
+				return nil, fmt.Errorf("%s: %q is not a reach the filter can read", m.Module, reach)
+			}
+			if e == nil {
+				e = map[int]string{}
+			}
+			e[port] = source
+		}
 		if e != nil {
 			exposure[m.Module] = e
 		}
@@ -1065,7 +1083,11 @@ func (r Resolution) contributions(settings SettingsBy, grants []Grant,
 			if err != nil {
 				return nil, fmt.Errorf("%s contributing to %s: %w", m.Module, to, err)
 			}
-			composeName(values, r.PublicDomain, r.At)
+			reaches, err := Reaches(m, settings[m.Module])
+			if err != nil {
+				return nil, fmt.Errorf("%s contributing to %s: %w", m.Module, to, err)
+			}
+			composeName(values, r.PublicDomain, r.At, reaches)
 			out[to] = append(out[to], Contribution{From: m.Module, Values: values})
 		}
 		// Several contributions to one requirement (ADR 0094's sibling for `contributes`): an
@@ -1079,7 +1101,11 @@ func (r Resolution) contributions(settings SettingsBy, grants []Grant,
 				if err != nil {
 					return nil, fmt.Errorf("%s contributing %s to %s: %w", m.Module, local, to, err)
 				}
-				composeName(values, r.PublicDomain, r.At)
+				reaches, err := Reaches(m, settings[m.Module])
+				if err != nil {
+					return nil, fmt.Errorf("%s contributing %s to %s: %w", m.Module, local, to, err)
+				}
+				composeName(values, r.PublicDomain, r.At, reaches)
 				out[to] = append(out[to], Contribution{From: m.Module, Values: values})
 			}
 		}
@@ -1110,10 +1136,34 @@ func (r Resolution) contributions(settings SettingsBy, grants []Grant,
 // the running mesh keeps serving the full names it has. And a labelled contribution on a node with
 // no public domain composes nothing — there is nothing to join it to — which reads downstream as a
 // route that named no host, the same as it would have before this existed.
-func composeName(values map[string]any, publicDomain, internalDomain string) {
+func composeName(values map[string]any, publicDomain, internalDomain string, reaches map[int]string) {
 	if values == nil {
 		return
 	}
+	// **How far the endpoint this route serves reaches decides which names exist** (novox/hq ADR
+	// 0138). Both were composed whenever the node had both domains, so every routed module got a
+	// public name and an internal one whether anybody wanted them or not — and a certificate for
+	// each, because the proxy certifies the names it is given.
+	//
+	// Joined by the port: a route entry names the port it serves and the module declares a listen on
+	// it. An entry with no port is not an endpoint's route but a rule about a name — a path-level
+	// refusal shadowing another route — and it inherits whatever that route's names turned out to
+	// be, which is why it is left alone here.
+	//
+	// Nothing said is both names, as before. That is what keeps every mesh already running identical
+	// until an assignment speaks.
+	wantPublic, wantInternal := true, true
+	if port, ok := asPort(values["port"]); ok {
+		if reach, said := reaches[port]; said {
+			wantPublic, wantInternal = WantsPublicName(reach), WantsInternalName(reach)
+		}
+	}
+	if !wantPublic {
+		publicDomain = ""
+	}
+	if !wantInternal {
+		internalDomain = ""
+	}
 	if _, already := values["name"]; already {
 		// A full name was given rather than a label. Left as-is: this is the legacy shape, and the
 		// point of the label is to not have to write the full name — a contribution that wrote both