[Bug] Muya inserts blank lines between adjacent bullet and task items

Description

Muya inserts a blank line between adjacent bullet and task items when serializing Markdown. This changes a tight list into a loose list even when no spacing change was requested.

Steps to Reproduce

Parse and serialize this Markdown with Muya (packages/muya):

- A bullet
- [ ] A task

Expected Behavior

The list should remain unchanged, with no blank line between its items.

Actual Behavior

Muya produces:

- A bullet

- [ ] A task

The same problem occurs with the task item first and with mixed lists nested inside an ordered item. Authored loose spacing should still be preserved.

Operating System

Linux (Other).

MarkText Version

Muya @muyajs/core 0.2.0 from develop, tested on 6 September 2026. This reproduction uses the engine directly, not a packaged desktop release.

OS Version

Debian 12 container.

Reproducibility

The automated serialization regressions reliably reproduce the problem on unchanged upstream code.

Related PR: #5254.

Start in packages/muya and run the automated serialization regressions described in the issue using the adjacent bullet and task example. Trace the Markdown serialization path and compare it with the related PR #5254; done means tight mixed lists remain unchanged while authored loose spacing is preserved, including reversed order and nested cases.