The scanner dropped every blank line within and after a block scalar, so
a literal block lost internal blank lines, a folded block turned a blank
line into a space rather than a newline, and the keep indicator could not
preserve trailing blank lines. Capture blank lines as empty content in
both the batch scan loop and the streaming TokenCursor loop, which had
drifted as duplicate copies. Rework blockScalarValue to apply literal
versus folded line semantics and clip/strip/keep chomping over the
captured lines; content without blank lines is unchanged. Re-pin the
multiline-scalar test whose expectation encoded the old dropped-blank
behavior, and add a chomping-and-folding test matrix.