Merge main into macros: resolve nav restructure conflicts

Take HEAD's updated typed-sx content (deftype, effect system details)
with main's /etc/plans/ path prefix. Take main's newer sx-browser.js
timestamp.

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

shared/sx/ref/eval.sx

@@ -91,7 +91,8 @@
             (= name "true")        true
             (= name "false")       false
             (= name "nil")         nil
-            :else                   (error (str "Undefined symbol: " name))))
+            :else                   (do (debug-log "Undefined symbol:" name "primitive?:" (primitive? name))
+                                      (error (str "Undefined symbol: " name)))))
 
       ;; --- keyword → its string name ---
       "keyword" (keyword-name expr)