A block sequence entry cannot follow a flow collection that closed earlier
on the same line ('- { y: z }- invalid', '- [a] - b'): the entry would
begin a sequence inline after a complete node. The scanner accepted it,
producing extra sequence items.
The scanner now records when a flow collection closes at the top level on
the current line and rejects a following block entry. The flag resets at
each line break, so a flow collection on its own line is unaffected, and a
nested block entry ('- -') has no preceding flow close, so it stays valid.
Measured against the official YAML test-suite, parse-level conformance
rises from 276/333 (82.9%) to 277/333 (83.2%) with no regression.
