A compact nested block sequence (`- - x`) carries no indent or dedent tokens,
only block entries at their source columns. The sibling-entry loop in
parseBlockSequence collected every following block entry by kind alone, so the
inner sequence absorbed an entry that belonged to an enclosing one: `- - x`
then `- - y` parsed as a single outer item `[[x, [y]]]` instead of two items
`[[x], [y]]`.
Record the first entry's column as the sequence's column and continue the
sibling loop only while the next entry sits at it. An entry at a smaller
column ends this sequence and is handled by the enclosing one; entries at the
same column still group, and flat sequences are unchanged.
Raises yaml-test-suite value-level conformance to 190/194 (97.9%) with the
parse-level and should-error rejection rates unchanged. 460 tests pass.
