Answer architecture round 1: one invariant phrase in internal/invariant, held across every package, Source built by Dir and FS, coverage counted across packages, and file items 112 and 113

internal/invariant/invariant.go

@@ -0,0 +1,9 @@
+// Package invariant spells what every package panics with when an invariant breaks.
+package invariant
+
+import "fmt"
+
+// Broken is the panic value for a broken invariant, one phrase every break greps to.
+func Broken(format string, a ...any) string {
+	return "fejkdata: internal error: " + fmt.Sprintf(format, a...)
+}