Bootstrap parser.sx to Python, add reactive runtime plan

Replace hand-written serialize/sx_serialize/sx_parse in Python with
spec-derived versions from parser.sx. Add parser as a Python adapter
alongside html/sx/async — all 48 parser spec tests pass.

Add reactive runtime plan to sx-docs: 7 feature layers (ref, foreign
FFI, state machines, commands with undo/redo, render loops, keyed
lists, client-first app shell) — zero new platform primitives.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/sx/ref/bootstrap_py.py

@@ -85,7 +85,12 @@ class PyEmitter:
                 if name == "define-async":
                     return self._emit_define_async(expr, indent)
                 if name == "set!":
-                    return f"{pad}{self._mangle(expr[1].name)} = {self.emit(expr[2])}"
+                    varname = expr[1].name if isinstance(expr[1], Symbol) else str(expr[1])
+                    py_var = self._mangle(varname)
+                    cell_vars = getattr(self, '_current_cell_vars', set())
+                    if py_var in cell_vars:
+                        return f"{pad}_cells[{self._py_string(py_var)}] = {self.emit(expr[2])}"
+                    return f"{pad}{py_var} = {self.emit(expr[2])}"
                 if name == "when":
                     return self._emit_when_stmt(expr, indent)
                 if name == "do" or name == "begin":
@@ -747,12 +752,12 @@ class PyEmitter:
         nested_set_vars = self._find_nested_set_vars(body)
         def_kw = "async def" if is_async else "def"
         lines = [f"{pad}{def_kw} {py_name}({params_str}):"]
-        if nested_set_vars:
-            lines.append(f"{pad}    _cells = {{}}")
         # Emit body with cell var tracking (and async context if needed)
         old_cells = getattr(self, '_current_cell_vars', set())
+        if nested_set_vars and not old_cells:
+            lines.append(f"{pad}    _cells = {{}}")
         old_async = self._in_async
-        self._current_cell_vars = nested_set_vars
+        self._current_cell_vars = old_cells | nested_set_vars
         if is_async:
             self._in_async = True
         self._emit_body_stmts(body, lines, indent + 1)