fix(md): Fix nested list termination on unindented paragraph

[JeanMertz]

When a nested list item was followed by a blank line and then a paragraph at column 0, the outer InList handler was folding that paragraph into the outer item as a lazy continuation instead of terminating the list.

The root cause was two-fold. First, flush_list_segment drained the exact bytes it emitted into the Block, which consumed the trailing blank line that carried the prev_blank=true termination signal. The parent state therefore never saw the blank and treated the following content as a continuation. Second, prev_blank was always initialised to false at the start of handle_in_list, even when a leading blank had already been stripped from the buffer head — so a child scope that consumed the blank during its own flush left the parent unaware.

The fix splits flush_list_segment's single flush_pos parameter into two: content_end (how many bytes go into the Block) and drain_end (how many bytes are removed from the buffer). For the Terminator branch, content_end = scan (includes trailing blanks, so the renderer preserves the visual separator) while drain_end = last_content_end (stops before the blanks, leaving them in the buffer for the popped-to parent state to pick up as prev_blank=true). For SiblingMarker and NestedContainer branches the two values are identical, preserving existing behaviour. prev_blank is now seeded from leading_blank > 0 so re-entry after a child scope consumed a blank fires the terminator check on the very first line.