Markdown nested list spread fix

Goal

Fix markdown serializer so indented nested lists stringify without blank lines.

Findings

• Added failing regression coverage in standardList.spec.tsx for nested indented lists; current output is "* parent\n\n * child\n".
• remark-stringify emits the blank line when nested listItem.spread is missing.
• Classic list serializer already sets listItem.spread: false; listToMdastTree needs the same mdast shape.

Plan

1. Add explicit spread to list items in listToMdastTree.ts, including block-id path.
2. Align helper specs to the normalized mdast shape.
3. Run serializer tests, package tests, build-first typecheck, and lint.