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

pins.go

@@ -1,6 +1,10 @@
 package fejkdata
 
-import "fmt"
+import (
+	"fmt"
+
+	"github.com/larvit/fejkdata/internal/invariant"
+)
 
 type tablePin struct {
 	t   *table
@@ -28,7 +32,7 @@ func (p *pinSet) pinned(t *table) (int, bool) {
 func (p *pinSet) mustRow(t *table) int {
 	r, ok := p.pinned(t)
 	if !ok {
-		panic(internalError("a column of %s is rendered with no row pinned", t.segment))
+		panic(invariant.Broken("a column of %s is rendered with no row pinned", t.segment))
 	}
 	return r
 }