Fix spec-explorer-data: pass metadata from SX routing instead of env lookup

The helper was trying to look up all-spec-items from get_component_env(),
but that only contains defcomp/defmacro — not regular defines. Now the
SX routing layer calls find-spec and passes filename/title/desc directly.

Also adds boundary declaration for spec-explorer-data.

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

sx/sxc/pages/docs.sx

@@ -352,9 +352,15 @@
         :else (cond
           (starts-with? slug "explore/")
             (let ((spec-slug (slice slug 8 (string-length slug)))
-                  (data (spec-explorer-data spec-slug)))
-              (if data
-                (~spec-explorer-content :data data)
+                  (spec (find-spec spec-slug)))
+              (if spec
+                (let ((data (spec-explorer-data
+                              (get spec "filename")
+                              (get spec "title")
+                              (get spec "desc"))))
+                  (if data
+                    (~spec-explorer-content :data data)
+                    (~spec-not-found :slug spec-slug)))
                 (~spec-not-found :slug spec-slug)))
           :else (let ((spec (find-spec slug)))
             (if spec