js-on-sx: ASI — :nl token flag + return restricted production (525/526 unit, 148/148 slice)

Lexer: adds :nl (newline-before) boolean to every token. scan! resets the flag
before each skip-ws! call; skip-ws! sets it true when it consumes \n or \r.
Parser: jp-token-nl? reads the flag; jp-parse-return-stmt stops before the
expression when a newline precedes it (return\n42 → return undefined). Four
new tests cover the restricted production and the raw flag.

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

lib/js/lexer.sx

@@ -94,7 +94,7 @@
   (fn
     (src)
     (let
-      ((tokens (list)) (pos 0) (src-len (len src)))
+      ((tokens (list)) (pos 0) (src-len (len src)) (nl-before false))
       (define
         js-peek
         (fn
@@ -109,11 +109,7 @@
           (let
             ((sl (len s)))
             (and (<= (+ pos sl) src-len) (= (slice src pos (+ pos sl)) s)))))
-      (define
-        js-emit!
-        (fn
-          (type value start)
-          (append! tokens (js-make-token type value start))))
+      (define js-emit! (fn (type value start) (append! tokens {:pos start :value value :type type :nl nl-before})))
       (define
         skip-line-comment!
         (fn
@@ -136,7 +132,13 @@
           ()
           (cond
             ((>= pos src-len) nil)
-            ((js-ws? (cur)) (do (advance! 1) (skip-ws!)))
+            ((js-ws? (cur))
+              (do
+                (when
+                  (or (= (cur) "\n") (= (cur) "\r"))
+                  (set! nl-before true))
+                (advance! 1)
+                (skip-ws!)))
             ((and (= (cur) "/") (< (+ pos 1) src-len) (= (js-peek 1) "/"))
               (do (advance! 2) (skip-line-comment!) (skip-ws!)))
             ((and (= (cur) "/") (< (+ pos 1) src-len) (= (js-peek 1) "*"))
@@ -568,6 +570,7 @@
         (fn
           ()
           (do
+            (set! nl-before false)
             (skip-ws!)
             (when
               (< pos src-len)