fix: keep assignments after a bare export line in the fast parser

When a line holds only `export`, the fast scanner's lookahead for the
key after `export` crosses the line break and lands on the next line.
If that line is itself `export KEY=value`, the scanner reads `export`
as the key, finds no separator, and skips the rest of that line, so
KEY is silently dropped. The classic parser keeps KEY=value.

Resume from the line that holds the `export` keyword instead, like the
separator lookahead already does for a key without a separator.