A `trailer` marker inside an unreferenced stream followed by `(` and more
than 4 KiB of text made the lexer stop at the window's end, so a later
`/Encrypt` name anywhere in the file refused a plain file. The unbounded
scan read a string there and opened it. The past-the-window fallback now
applies only when no object starts in the window or the first one opens
a dictionary (`<<`, including one split by the window's end).
Regression test with `trailer(` and `trailer[` in a stream and a later
Encrypt name.