Add js.sx bootstrapper docs page with G0 bug discovery writeup

Documents the self-hosting process for js.sx including the G0 bug
where Python's `if fn_expr` treated 0/False/"" as falsy, emitting
NIL instead of the correct value. Adds live verification page,
translation differences table, and nav entry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

shared/sx/ref/bootstrap_js.py

@@ -872,7 +872,7 @@ class JSEmitter:
             body = fn_expr[2:]
             loop_body = self._emit_loop_body(name, body)
             return f"var {self._mangle(name)} = function() {{ while(true) {{ {loop_body} }} }};"
-        val = self.emit(fn_expr) if fn_expr else "NIL"
+        val = self.emit(fn_expr) if fn_expr is not None else "NIL"
         return f"var {self._mangle(name)} = {val};"
 
     def _is_self_tail_recursive(self, name: str, body: list) -> bool: