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-sx.sx

@@ -66,6 +66,10 @@
             (starts-with? name "~")
               (aser-call name args env)
 
+            ;; Lake — serialize (server-morphable slot)
+            (= name "lake")
+              (aser-call name args env)
+
             ;; HTML tag — serialize
             (contains? HTML_TAGS name)
               (aser-call name args env)