Add test case for MyST target anchors before title heading

[mmcky]

Problem

The parseDocumentComponents() method in src/parser.ts expects a # Title heading immediately after frontmatter. However, MyST documents often have target anchors before the title:

---
jupytext:
  ...
---

(about_py)=           <-- Target anchor
\```{raw} jupyter   <-- Raw block  
...
\```

# About These Lectures  <-- Title is HERE, not immediately after frontmatter

This causes parseDocumentComponents() to throw:

Expected # title heading at line 12, found: (about_py)=

Current Impact

• Bulk Translator: Fixed by switching to parseSections() for heading-map generation
• Incremental Sync (file-processor.ts): May fail on source files with this structure

Requested Changes

1. Add test cases to src/__tests__/parser-components.test.ts for documents with:

   • Target anchors (anchor-name)= before title
   • Raw blocks before title
   • Index directives before title
   • Combination of the above (like real QuantEcon lectures)

2. Update parseDocumentComponents() to handle content between frontmatter and title:

   • Skip target anchors (...)=
   • Skip MyST directives before title
   • Find the # Title heading wherever it appears

Test Fixture

Use the structure from lecture-python-programming.myst:

---
jupytext:
  text_representation:
    extension: .md
    format_name: myst
kernelspec:
  display_name: Python 3
  language: python
  name: python3
---

(about_py)=
\```{raw} jupyter
<div id="qe-notebook-header">...</div>
\```

\```{index} single: python
\```

# About These Lectures

Related

• Discovered while testing bulk translator with Persian translation
• The fix for bulk translator is in place (uses parseSections() instead)