Content-addressed computation: freeze → hash → CID → thaw

Hash frozen SX to a content identifier (djb2 → hex). Same state
always produces the same CID. Store by CID, retrieve by CID.

- content-hash: djb2 hash of SX text → hex string
- content-put/get: in-memory content store
- freeze-to-cid: freeze scope → store → return CID
- thaw-from-cid: look up CID → thaw signals
- char-code-at / to-hex primitives for both platforms
- Live demo: counter + name widget, content-address button,
  CID display, restore from CID input, CID history

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/geography/cek.sx

@@ -478,6 +478,73 @@
 ;; CEK Freeze / Thaw — serializable computation
 ;; ---------------------------------------------------------------------------
 
+(defisland ~geography/cek/content-address-demo ()
+  (let ((count (signal 0))
+        (name (signal "world"))
+        (cid-display (signal ""))
+        (cid-input (signal ""))
+        (cid-history (signal (list)))
+        (status (signal "")))
+    ;; Register in freeze scope
+    (freeze-scope "ca-demo" (fn ()
+      (freeze-signal "count" count)
+      (freeze-signal "name" name)))
+    (div (~cssx/tw :tokens "space-y-4")
+      ;; Interactive widget
+      (div (~cssx/tw :tokens "flex gap-4 items-center")
+        (div (~cssx/tw :tokens "flex items-center gap-2")
+          (button :on-click (fn (e) (swap! count dec))
+            (~cssx/tw :tokens "px-2 py-1 rounded bg-stone-200 text-stone-700 hover:bg-stone-300") "-")
+          (span (~cssx/tw :tokens "font-mono text-lg font-bold w-8 text-center") (deref count))
+          (button :on-click (fn (e) (swap! count inc))
+            (~cssx/tw :tokens "px-2 py-1 rounded bg-stone-200 text-stone-700 hover:bg-stone-300") "+"))
+        (input :type "text" :bind name
+          (~cssx/tw :tokens "px-3 py-1 rounded border border-stone-300 font-mono text-sm")))
+      ;; Live output
+      (div (~cssx/tw :tokens "rounded bg-violet-50 border border-violet-200 p-3 text-violet-800")
+        (str "Hello, " (deref name) "! Count = " (deref count)))
+      ;; Content address button
+      (div (~cssx/tw :tokens "flex gap-2")
+        (button :on-click (fn (e)
+          (let ((cid (freeze-to-cid "ca-demo")))
+            (reset! cid-display cid)
+            (reset! status (str "Stored as " cid))
+            (swap! cid-history (fn (h) (append h (list cid))))))
+          (~cssx/tw :tokens "px-3 py-1.5 rounded bg-stone-700 text-white text-sm hover:bg-stone-800")
+          "Content-address"))
+      ;; CID display
+      (when (not (empty? (deref cid-display)))
+        (div (~cssx/tw :tokens "font-mono text-sm bg-stone-50 rounded p-2 flex items-center gap-2")
+          (span (~cssx/tw :tokens "text-stone-400") "CID:")
+          (span (~cssx/tw :tokens "text-violet-700 font-bold") (deref cid-display))))
+      ;; Status
+      (when (not (empty? (deref status)))
+        (div (~cssx/tw :tokens "text-xs text-emerald-600") (deref status)))
+      ;; Restore from CID
+      (div (~cssx/tw :tokens "flex gap-2 items-end")
+        (div (~cssx/tw :tokens "flex-1")
+          (label (~cssx/tw :tokens "text-xs text-stone-400 block mb-1") "Restore from CID")
+          (input :type "text" :bind cid-input
+            (~cssx/tw :tokens "w-full px-3 py-1 rounded border border-stone-300 font-mono text-sm")))
+        (button :on-click (fn (e)
+          (if (thaw-from-cid (deref cid-input))
+            (reset! status (str "Restored from " (deref cid-input)))
+            (reset! status (str "CID not found: " (deref cid-input)))))
+          (~cssx/tw :tokens "px-3 py-1.5 rounded bg-emerald-600 text-white text-sm hover:bg-emerald-700")
+          "Restore"))
+      ;; CID history
+      (when (not (empty? (deref cid-history)))
+        (div (~cssx/tw :tokens "space-y-1")
+          (label (~cssx/tw :tokens "text-xs text-stone-400 block") "History")
+          (map (fn (cid)
+            (button :on-click (fn (e)
+              (if (thaw-from-cid cid)
+                (reset! status (str "Restored from " cid))
+                (reset! status (str "CID not found: " cid))))
+              (~cssx/tw :tokens "block w-full text-left px-2 py-1 rounded bg-stone-50 hover:bg-stone-100 font-mono text-xs text-stone-600")
+              cid))
+            (deref cid-history)))))))
+
 (defcomp ~geography/cek/cek-freeze-content ()
   (~docs/page :title "Freeze / Thaw"
 
@@ -525,6 +592,16 @@
          "The frozen SX appears below. Click Thaw to resume from the frozen state.")
       (~geography/cek/freeze-demo))
 
+    (~docs/section :title "Content addressing" :id "content-addressing"
+      (p "Hash the frozen SX " (~docs/inline-code "\u2192") " content identifier. "
+         "Same state always produces the same CID. Store by CID, retrieve by CID, verify by CID.")
+      (~docs/code :code (highlight
+        "(freeze-to-cid \"widget\")\n;; => \"d9eea67b\"\n\n(thaw-from-cid \"d9eea67b\")\n;; Signals restored. Same CID = same state."
+        "lisp"))
+      (p "Try it: change the values, click Content-address. Copy the CID. "
+         "Change the values again. Paste the CID and Restore.")
+      (~geography/cek/content-address-demo))
+
     (~docs/section :title "What this enables" :id "enables"
       (ul :class "list-disc pl-6 mb-4 space-y-2 text-stone-600"
         (li (strong "Persistence") " \u2014 save reactive island state to localStorage, "