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/html.py

@@ -469,6 +469,38 @@ def _render_island(island: Island, args: list, env: dict[str, Any]) -> str:
     return "".join(parts)
 
 
+def _render_lake(args: list, env: dict[str, Any]) -> str:
+    """Render a server-morphable lake slot.
+
+    (lake :id "name" :tag "div" children...)
+    → <div data-sx-lake="name">children</div>
+
+    Lakes are server territory inside reactive islands. During morph,
+    the server can update lake content while surrounding reactive DOM
+    is preserved.
+    """
+    lake_id = ""
+    lake_tag = "div"
+    children: list[Any] = []
+    i = 0
+    while i < len(args):
+        arg = args[i]
+        if isinstance(arg, Keyword) and i + 1 < len(args):
+            kname = arg.name
+            kval = _eval(args[i + 1], env)
+            if kname == "id":
+                lake_id = str(kval) if kval is not None and kval is not NIL else ""
+            elif kname == "tag":
+                lake_tag = str(kval) if kval is not None and kval is not NIL else "div"
+            i += 2
+        else:
+            children.append(arg)
+            i += 1
+
+    body = "".join(_render(c, env) for c in children)
+    return f'<{lake_tag} data-sx-lake="{_escape_attr(lake_id)}">{body}</{lake_tag}>'
+
+
 def _render_list(expr: list, env: dict[str, Any]) -> str:
     """Render a list expression — could be an HTML element, special form,
     component call, or data list."""
@@ -494,6 +526,10 @@ def _render_list(expr: list, env: dict[str, Any]) -> str:
         if name == "<>":
             return "".join(_render(child, env) for child in expr[1:])
 
+        # --- lake → server-morphable slot within island -------------------
+        if name == "lake":
+            return _render_lake(expr[1:], env)
+
         # --- html: prefix → force tag rendering --------------------------
         if name.startswith("html:"):
             return _render_element(name[5:], expr[1:], env)