dream: hello + counter demos + 10 end-to-end tests

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/dream/demos/counter.sx

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+;; lib/dream/demos/counter.sx — per-session visit counter (counter.ml).
+;; Demonstrates the session middleware: each browser session keeps its own count.
+
+(define
+  dream-counter-handler
+  (fn
+    (req)
+    (let
+      ((n (+ 1 (or (dream-session-field req "count") 0))))
+      (begin
+        (dream-set-session-field req "count" n)
+        (dream-html (str "<p>You have visited this page " n " time(s).</p>"))))))
+
+;; reset clears the session counter
+(define
+  dream-counter-reset
+  (fn
+    (req)
+    (begin
+      (dream-set-session-field req "count" 0)
+      (dream-redirect "/"))))
+
+(define
+  dream-counter-app-with
+  (fn
+    (backend)
+    ((dream-sessions backend)
+      (dream-router
+        (list
+          (dream-get "/" dream-counter-handler)
+          (dream-post "/reset" dream-counter-reset))))))
+
+(define dream-counter-app (dream-counter-app-with (dream-memory-sessions)))
+
+;; entry point: (dream-run (dream-counter-app-with (dream-memory-sessions)))