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>

node.go

@@ -267,7 +267,7 @@ var reservedList = strings.Join(strings.Split(reservedInName, ""), " ")
 // name may contain.
 func checkName(name string) error {
 	if name == "" {
-		return fmt.Errorf("%q is empty, so no dot path can reach it", name)
+		return fmt.Errorf("%q is empty, which is not a path segment, so List never offers it", name)
 	}
 	if i := strings.IndexAny(name, reservedInName); i >= 0 {
 		return fmt.Errorf("%q contains %q; a name may not use %s, which the dot path and {token} grammars reserve",