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>

sx/sx/boundary.sx

@@ -5,6 +5,11 @@
   :returns "sx-source"
   :service "sx")
 
+(define-page-helper "component-source"
+  :params (name)
+  :returns "string"
+  :service "sx")
+
 (define-page-helper "primitives-data"
   :params ()
   :returns "dict"