Say why an empty name is rejected, and finish what that removed

The reason given was wrong. A dot path did reach an empty-named field:
Fake("a.") returned it, Fake("a.b.") returned a nested one, and {..a.} bound
it. What was true is narrower — List never offered it, because an empty name
is no path segment — so the engine accepted spellings it would not advertise.
The message, the test comment and the README say that instead.

Two things the rejection finished off:

A {} token still reported "no field \"\"", pointing at a fix the loader now
rejects — two errors for one rule. It is told the name can never exist, like
an option token already is.

addressable is dead: both halves of "not empty and no dot" are now rejected
where a name is authored. Mutating it to panic leaves the suite green here
and panics on main, so it was live and is not. Keeping it would preserve the
silent-hiding this change removes — List quietly omitting a name rather than
New refusing it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

edge_test.go

@@ -120,9 +120,9 @@ func TestNewErrors(t *testing.T) {
 			map[string]string{"a|b": `["1"]`},
 			`category "a|b" contains "|"`,
 		},
-		// An empty name is reachable by no dot path, so List cannot advertise it and
-		// Fake cannot ask for it — {} is the one spelling that reaches it, which is
-		// the same trap {a.} is already rejected for.
+		// An empty name is not a path segment, so List never offered it — while a
+		// bare {}, a trailing dot in Fake("a.") and a {..a.} reference all reached
+		// it. The engine accepted spellings it would never advertise.
 		"empty field name": {
 			map[string]string{"a": `{"format":"[{}]","":"VALUE"}`},
 			`field "" is empty`,
@@ -156,9 +156,15 @@ func TestNewErrors(t *testing.T) {
 			map[string]string{"a": `{"format":"{..|..}"}`},
 			"reference has no path",
 		},
+		// No field can be named "", so the token is told that rather than sent to
+		// name one — the fix "no field" points at is itself a load error.
 		"repeated empty arm": {
 			map[string]string{"a": `{"format":"{|}"}`},
-			`no field ""`,
+			"a name is never empty",
+		},
+		"bare empty token": {
+			map[string]string{"a": `{"format":"[{}]","x":["1"]}`},
+			"a name is never empty",
 		},
 	}
 	for name, c := range rejected {