Level 2-3: lake morphing — server content flows through reactive islands

Lake tag (lake :id "name" children...) creates server-morphable slots
within islands. During morph, the engine enters hydrated islands and
updates data-sx-lake elements by ID while preserving surrounding
reactive DOM (signals, effects, event listeners).

Specced in .sx, bootstrapped to JS and Python:
- adapter-dom.sx: render-dom-lake, reactive-attr marks data-sx-reactive-attrs
- adapter-html.sx: render-html-lake SSR output
- adapter-sx.sx: lake serialized in wire format
- engine.sx: morph-island-children (lake-by-ID matching),
  sync-attrs skips reactive attributes
- ~sx-header uses lakes for logo and copyright
- Hegelian essay updated with lake code example

Also includes: lambda nil-padding for missing args, page env ordering fix

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

shared/sx/ref/eval.sx

@@ -217,14 +217,18 @@
   (fn (f args caller-env)
     (let ((params (lambda-params f))
           (local  (env-merge (lambda-closure f) caller-env)))
-      (if (!= (len args) (len params))
+      ;; Too many args is an error; too few pads with nil
+      (if (> (len args) (len params))
         (error (str (or (lambda-name f) "lambda")
                     " expects " (len params) " args, got " (len args)))
         (do
-          ;; Bind params
+          ;; Bind params — provided args first, then nil for missing
           (for-each
             (fn (pair) (env-set! local (first pair) (nth pair 1)))
             (zip params args))
+          (for-each
+            (fn (p) (env-set! local p nil))
+            (slice params (len args)))
           ;; Return thunk for TCO
           (make-thunk (lambda-body f) local))))))