host: per-block guarded render — editor posts never 502, real prose shows

A post created with the editor stored sx_content containing components the host
can't resolve: the legacy editor emits bare ~kg-md while the cards are
~kg_cards/kg-md (drift — not papered over with aliases). render-to-html threw on
the undefined symbol and host/blog-render had no error handling -> handler crash
-> 502 on a REAL post (/mddddd/).

Fix: render each block of the (<> ...) fragment under its own guard via
render-page (env-supplied). Real prose (p/h1/ul/...) renders; an unsupported or
malformed block degrades to a <div class=blk-unsupported> placeholder; a bad
block never crashes the handler. Verified live: /mddddd/ + all junk posts now
200 (text shown, cards placeheld). Full kg-card rendering = follow-on (resolve
the name drift / native editor).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/host/blog.sx

@@ -59,12 +59,30 @@
       (host/blog-slugs))))
 
 ;; ── render ──────────────────────────────────────────────────────────
-;; A post's sx_content is SX element markup -> HTML via the component renderer.
+;; A post's sx_content is SX element markup -> HTML via render-page (which supplies
+;; the server env so components resolve + keyword attrs are kept).
+;;
+;; Rendered PER BLOCK and guarded: the editor wraps content in a (<> ...) fragment
+;; of blocks, some of which the host can't render (the legacy editor emits bare
+;; ~kg-md cards while the components are ~kg_cards/kg-md — drift we don't paper over
+;; with aliases). Rendering each block under its own guard means the real prose
+;; (p/h1/ul/...) shows and only the unsupported block degrades to a placeholder —
+;; and a bad block never crashes the handler (-> 502).
+(define host/blog--render-node
+  (fn (node)
+    (guard (e (true "<div class=\"blk-unsupported\"><em>(unsupported block)</em></div>"))
+      (render-page node))))
 (define host/blog-render
   (fn (record)
     (let ((sx (get record :sx-content)))
       (if (and sx (not (= sx "")))
-        (render-to-html (parse sx))
+        (let ((tree (guard (e (true nil)) (parse sx))))
+          (cond
+            ((nil? tree) "<p><em>(unparseable content)</em></p>")
+            ((and (= (type-of tree) "list") (> (len tree) 0)
+                  (= (str (first tree)) "<>"))
+              (join "" (map host/blog--render-node (rest tree))))
+            (else (host/blog--render-node tree))))
         (str "<p>(empty post)</p>")))))
 (define host/blog--page
   (fn (title body)