Step a table's routes past its selector, and panic on an arm with no bound head

docs/decisions.md

@@ -846,9 +846,8 @@ the direct chain is the one a reader can predict from the tables' parents.
 
 2026-09-28, larv-review on systems-architect's finding; approved 2026-09-28 by lilleman.
 Re-tested 2026-09-29 by collapsing them into one loop switching on a mode field.
-Goals 2 and 11: `pathCheck`, `pathProbe` and `pathDraw` stay separate loops; `pathCover`
-became the cover `pathCheck` compiles, on 2026-09-30.
-One loop fails `perf_test.go`: the check's leaves, the probe's selectors and the errors
+Goals 2 and 11: `pathCheck`, `pathProbe` and `pathDraw` stay separate loops.
+One loop fails `perf_test.go`: the check's leaves and the errors
 naming a level leak the walker's contents, and Go tracks a struct's fields as one, so
 the draw's pins, arm and memo leak with them. Held by pointer, a repeat of a reference
 path rose from 66 to 106 allocations; copied into the walker by value,

path.go

@@ -278,6 +278,7 @@ func (w *pathCheck) tableSteps(t *table, r tableRoute, tail []string) error {
 		}
 		row, _ := w.pins.pinned(t)
 		w.emit(pathStep{kind: stepSelect, name: r.sel, row: row}, tail)
+		tail = tail[1:]
 	}
 	if r.draw {
 		w.emit(pathStep{kind: stepDraw}, tail)

reference.go

@@ -113,10 +113,12 @@ func compileArms(t *template) {
 }
 
 func compileArm(t *template, a *arm) {
-	if head := t.head(a.head); head != nil {
-		w := compilePath(head, a.tail)
-		a.steps, a.leaves, a.cover = w.steps, w.leaves, w.cover
+	head := t.head(a.head)
+	if head == nil {
+		panic(internalError("{%s} reads a head nothing bound", a.spelling))
 	}
+	w := compilePath(head, a.tail)
+	a.steps, a.leaves, a.cover = w.steps, w.leaves, w.cover
 }
 
 // resolveLink binds every reference t reads, refusing one to t's own category, and keys