tcl: Phase 6a fix vwait :: routing — was infinite-looping

vwait used frame-lookup which doesn't honor `::` global routing. So
`vwait ::done` after `set ::done fired` (where set routes to root frame)
never saw the var change in the local frame, looping forever.

Added tcl-vwait-lookup helper that mirrors tcl-var-get's `::` routing
but returns nil instead of erroring on missing vars.

Was the deadlock that hung the full test suite past test 32.

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

lib/tcl/runtime.sx

@@ -3544,6 +3544,22 @@
           (tcl-event-step interp (- target-ms now))
           target-ms)))))
 
+; 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
+  (fn
+    (interp name)
+    (if
+      (tcl-global-ref? name)
+      (let
+        ((root-frame
+            (let ((stack (get interp :frame-stack)))
+              (if (= 0 (len stack)) (get interp :frame) (first stack))))
+          (gname (tcl-strip-global name)))
+        (frame-lookup root-frame gname))
+      (frame-lookup (get interp :frame) name))))
+
 (define
   tcl-cmd-vwait
   (fn
@@ -3554,7 +3570,7 @@
       (let
         ((name (first args)))
         (let
-          ((initial (frame-lookup (get interp :frame) name)))
+          ((initial (tcl-vwait-lookup interp name)))
           (assoc (tcl-vwait-loop interp name initial) :result ""))))))
 
 (define
@@ -3562,7 +3578,7 @@
   (fn
     (interp name initial)
     (let
-      ((cur (frame-lookup (get interp :frame) name)))
+      ((cur (tcl-vwait-lookup interp name)))
       (if
         (and (not (nil? cur)) (not (equal? cur initial)))
         interp