Stop a settings layer replacing what a provider serves (issue 696)

What a provider serves is its definition's (ADR 0348 clause 11): a layer
fills only the keys it asks for as ${setting:<key>}. Settle replaced any
served key a layer named, so a setting for one of a provider's files could
change what every consumer was told. influxdb on ace was the one live layer
relying on it and now serves its settings (mesh-catalog, merged first), so
grafana's data source composes byte for byte as before. UnusedSettings no
longer counts a literal served key as a place a setting lands.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/catalogue/settings_test.go

@@ -190,22 +190,6 @@ func TestASettingLandsOnlyWhereSomethingDeclaresIt(t *testing.T) {
 	}
 }
 
-func TestASettingOverridesAServedKeyAndAddsNone(t *testing.T) {
-	// What a consumer is told is the provider's contract. A setting made for one of the
-	// provider's files — its site name, its public address — is not part of it.
-	served, err := Settle(map[string]any{"host": "mail", "port": 25},
-		[]Layer{{From: "the mesh", Values: map[string]any{"host": "post", "sitename": "Mail"}}})
-	if err != nil {
-		t.Fatal(err)
-	}
-	if served["host"] != "post" {
-		t.Errorf("the setting did not override the served host: %v", served)
-	}
-	if _, leaked := served["sitename"]; leaked {
-		t.Errorf("a setting for a file reached the consumers: %v", served)
-	}
-}
-
 func TestContentThatIsNotJSONIsRefusedWhereSomebodyIsLooking(t *testing.T) {
 	// Rather than on the machine, at apply time, as a file the program cannot read.
 	_, err := ApplySettings(file(`this is not json`), nil)