fix: reject bare import bindings

docs/spec.md

@@ -323,11 +323,25 @@ ImportPath    = ident { "." ident } .
   package declares `package wire;`, the importing file sees its exported names
   as `wire.Name`; `codec.Name` is not an additional binding. An explicit alias
   replaces only that visible qualifier: `import stable acme.codec;` exposes
-  `stable.Name`, not `wire.Name` or `codec.Name`. Both kinds of binding are
-  scoped to that source file. A sibling file must declare its own import.
+  `stable.Name`, not `wire.Name` or `codec.Name`. Each effective default or
+  explicit named binding is a file-local package-name object usable only as
+  the left qualifier of a selector. A bare occurrence in a value context is
+  rejected as `use of package BINDING not in selector`; a bare occurrence in
+  a type context is rejected as `BINDING (package name) is not a type`. Such a
+  rejected bare occurrence does not count as use of the import for
+  unused-import accounting. A legal selector does count as use, but does not
+  excuse any separate bare occurrence. Both kinds of named binding are scoped
+  to that source file. A sibling file must declare its own import; without one,
+  its otherwise equal spelling follows ordinary undefined-name or unknown-type
+  lookup rather than package-name diagnostics. Builtin spelling does not alter
+  the object: an import bound as `len`, `size`, `align`, or another builtin name
+  remains a selector-only package-name object.
   Neither form exposes an imported declaration as a bare `Name`; ordinary
   unqualified lookup remains limited to lexical, builtin, and same-package
-  declarations.
+  declarations. A blank import creates no package-name object, an effective
+  `init` import is rejected before installing one, and a missing target fails
+  during import resolution, so none of those cases acquires the bare-package
+  diagnostics or satisfies a named import's unused accounting.
 - An import whose effective file-local qualifier is `init` is invalid. This
   includes both `import init acme.codec;` and an unaliased import whose target
   declares `package init;`. Each resolved occurrence is rejected at its first