tcl: rename tcl-vwait-lookup → tcl-var-lookup-or-nil; use in info exists

Generalized helper for var-lookup-with-:: so info exists also works on
::-prefixed names.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/tcl/runtime.sx

@@ -3096,7 +3096,7 @@
             (let
               ((varname (first rest-args)))
               (let
-                ((val (frame-lookup (get interp :frame) varname)))
+                ((val (tcl-var-lookup-or-nil interp varname)))
                 (assoc interp :result (if (nil? val) "0" "1")))))
           ((equal? sub "hostname") (assoc interp :result "localhost"))
           ((equal? sub "script") (assoc interp :result ""))
@@ -3547,7 +3547,7 @@
 ; Look up a Tcl var by name, returning nil instead of erroring if missing.
 ; Handles `::var` global-prefix routing the same way tcl-var-get does.
 (define
-  tcl-vwait-lookup
+  tcl-var-lookup-or-nil
   (fn
     (interp name)
     (if
@@ -3570,7 +3570,7 @@
       (let
         ((name (first args)))
         (let
-          ((initial (tcl-vwait-lookup interp name)))
+          ((initial (tcl-var-lookup-or-nil interp name)))
           (assoc (tcl-vwait-loop interp name initial) :result ""))))))
 
 (define
@@ -3578,7 +3578,7 @@
   (fn
     (interp name initial)
     (let
-      ((cur (tcl-vwait-lookup interp name)))
+      ((cur (tcl-var-lookup-or-nil interp name)))
       (if
         (and (not (nil? cur)) (not (equal? cur initial)))
         interp