ENG-note: notHandedWhy's impossible branch says it is a fault in the node-engine's composition (review of issue 1966)

Claude-Session: https://claude.ai/code/session_01JG73KJNrH5EgFXMKKYSrig

internal/compose/resolution/handed.go

@@ -161,7 +161,11 @@ func notHandedWhy(with mcat.Rendering, m mcat.Manifest, r map[string]any) (why s
 	if why := machineWhy(with); why != "" {
 		return why, true
 	}
-	return fmt.Sprintf("its build kept a template for %s and this composition filled it itself", id), true
+	// Every way a handed file is not handed is named above: the module moved with templates, a template for this file, no
+	// merge, a machine that takes it. Reached, the composition and Handing disagree, and that is a fault in this code, said as
+	// one so it is reported and not read as a reason about the module or the machine (issue 1966).
+	return fmt.Sprintf("a fault in the node-engine's composition, to be reported: %s is moved, its build kept a template for %s, "+
+		"and this machine takes handed files, yet the composition filled it itself", m.Module, id), true
 }
 
 // requiredByWords begins why a module only pulled in by another is on a machine (Resolution.Because): `required by

internal/compose/resolution/handed_bound_test.go

@@ -157,4 +157,12 @@ func TestAHandedModuleNamingARunTimeBoundKeyComposesOnlyWhereItIsHanded(t *testi
 			t.Fatalf("%s: the unmarked module is refused %v", name, before)
 		}
 	}
+
+	// And where nothing keeps it from being handed yet the composition filled the file itself, the reason says it is a fault
+	// in this code (the review of the vault's judgement, issue 1966), never a reason about the module or the machine.
+	whole := mcat.Manifest{Module: "bot", Moved: true, Handed: &mcat.Handed{Templates: map[string]string{"f": "templates/f"}}}
+	if why, handedFile := notHandedWhy(handingTo("store:5000"), whole, map[string]any{"id": "f"}); !handedFile ||
+		!strings.Contains(why, "a fault in the node-engine's composition") {
+		t.Errorf("the impossible branch says %q (handed file %v)", why, handedFile)
+	}
 }