fix(stdio): correct position tracking on mode switch and free owned buffers on close

src/stdio/stdio_impl.h

@@ -17,9 +17,11 @@
  *
  * Mode switching: reading while write data is pending flushes it first;
  * writing while unread buffered data is pending seeks the kernel back to
- * the logical position (discarding the unread data). F_PUSHED marks the
- * single guaranteed ungetc pushback byte in `ungot`; a successful seek
- * discards it.
+ * the logical position (discarding the unread data). F_WRITE doubles as
+ * the current-mode marker and is cleared by the read switch; F_RDWR
+ * records the write capability of update streams so the write switch can
+ * re-set F_WRITE. F_PUSHED marks the single guaranteed ungetc pushback
+ * byte in `ungot`; a successful seek discards it.
  *
  * The helper prototypes below are shared with the formatted-I/O todos
  * (16: vfprintf, 17: vfscanf) so they operate on the same buffer state.
@@ -55,6 +57,7 @@ struct vlibc_FILE
 #define F_OWNBUF 0x080  /* buf was allocated by the library */
 #define F_HEAP 0x100    /* the FILE struct itself is heap-allocated */
 #define F_PUSHED 0x200  /* ungot holds a pushed-back byte */
+#define F_RDWR 0x400    /* update stream: reads and writes both enabled */
 
 /* Flush pending write data; returns 0, or -1 with F_ERR set. */
 hidden int