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/adapter-html.sx

@@ -81,6 +81,10 @@
               (= name "raw!")
                 (join "" (map (fn (x) (str (trampoline (eval-expr x env)))) args))
 
+              ;; Lake — server-morphable slot within an island
+              (= name "lake")
+                (render-html-lake args env)
+
               ;; HTML tag
               (contains? HTML_TAGS name)
                 (render-html-element name args env)
@@ -293,6 +297,43 @@
             "</" tag ">"))))))
 
 
+;; --------------------------------------------------------------------------
+;; render-html-lake — SSR rendering of a server-morphable slot
+;; --------------------------------------------------------------------------
+;;
+;; (lake :id "name" children...) → <div data-sx-lake="name">children</div>
+;;
+;; Lakes are server territory inside islands. The morph can update lake
+;; content while preserving surrounding reactive DOM.
+
+(define render-html-lake
+  (fn (args env)
+    (let ((lake-id nil)
+          (lake-tag "div")
+          (children (list)))
+      (reduce
+        (fn (state arg)
+          (let ((skip (get state "skip")))
+            (if skip
+              (assoc state "skip" false "i" (inc (get state "i")))
+              (if (and (= (type-of arg) "keyword")
+                       (< (inc (get state "i")) (len args)))
+                (let ((kname (keyword-name arg))
+                      (kval (trampoline (eval-expr (nth args (inc (get state "i"))) env))))
+                  (cond
+                    (= kname "id")  (set! lake-id kval)
+                    (= kname "tag") (set! lake-tag kval))
+                  (assoc state "skip" true "i" (inc (get state "i"))))
+                (do
+                  (append! children arg)
+                  (assoc state "i" (inc (get state "i"))))))))
+        (dict "i" 0 "skip" false)
+        args)
+      (str "<" lake-tag " data-sx-lake=\"" (escape-attr (or lake-id "")) "\">"
+           (join "" (map (fn (c) (render-to-html c env)) children))
+           "</" lake-tag ">"))))
+
+
 ;; --------------------------------------------------------------------------
 ;; render-html-island — SSR rendering of a reactive island
 ;; --------------------------------------------------------------------------