fix: prevent metadata loss in multi-threaded md2md builds

[martyanovandrey]

fix: prevent metadata loss when file is both entry and include

Problem

When building documentation in md2md mode, files that are both a TOC entry and included by another file could lose their metadata (like __system variables).

This issue is especially noticeable in multi-threaded builds, where processing order is non-deterministic. The metadata loss appears random — sometimes present, sometimes missing — depending on which thread writes last.

Root Cause

The issue occurs because the same file can be written twice:

1. As an entry — written WITH YAML frontmatter (metadata included)
2. As an include dependency — written WITHOUT YAML frontmatter

The last write wins. If the include write happens after the entry write, all metadata is lost.

Example scenario:

entry-as-include.md  ← listed in toc.yaml as entry
includer.md          ← contains {% include [](entry-as-include.md) %}

Processing order:

1. Process entry-as-include.md → write WITH frontmatter ✓
2. Process includer.md → write entry-as-include.md as include WITHOUT frontmatter ✗
   (overwrites the file, metadata lost)

In single-threaded mode, the order is deterministic (though still incorrect for affected files). In multi-threaded mode, the order depends on thread scheduling, making the bug appear intermittent.

How to Reproduce

1. Create a file that is both:

   • Listed in toc.yaml as an entry
   • Included via {% include %} in another file

2. Build with --output-format=md --add-system-meta

3. Check output — the file may be missing __system metadata depending on processing order

Solution

Add YAML frontmatter when writing include files in output-md:

Now both writers (entry and include) produce the same output with frontmatter, so the write order no longer matters.

[Feverqwe]

Looks grate!