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:

shared/sx/ref/js.sx

@@ -1305,12 +1305,7 @@
                   (symbol-name (nth expr 1))
                   (str (nth expr 1))))
           (val-expr (nth expr 2)))
-      ;; Match G0 bootstrap_js.py: Python `if fn_expr` treats 0/false/None/""
-      ;; as falsy, so (define x 0) emits NIL instead of 0. Reproduce for byte-match.
-      (if (or (nil? val-expr)
-              (and (= (type-of val-expr) "number") (= val-expr 0))
-              (and (= (type-of val-expr) "boolean") (= val-expr false))
-              (and (= (type-of val-expr) "string") (= val-expr "")))
+      (if (nil? val-expr)
         (str "var " (js-mangle name) " = NIL;")
         ;; Detect zero-arg self-tail-recursive functions → while loops
         (if (and (list? val-expr)