feat: add basic markdown syntax test files

Add comprehensive test suite for basic markdown syntax features:
- basic-headings.md: All heading levels and setext syntax
- basic-paragraphs-line-breaks.md: Paragraph separation and line breaks
- basic-emphasis.md: Bold and italic formatting
- basic-blockquotes.md: Blockquotes with nested content
- basic-lists.md: Ordered and unordered lists with nesting
- basic-code.md: Inline code and code blocks
- basic-horizontal-rules.md: Horizontal rule variants
- basic-links-and-images.md: Links and images with various syntax
- basic-escaping.md: Character escaping
- basic-index.md: Index of all test files

All files follow the Markdown Guide's Basic Syntax specification.

test/markdown/basic-horizontal-rules.md

@@ -0,0 +1,190 @@
+# Basic Horizontal Rules Test
+
+This file tests horizontal rule syntax using hyphens, asterisks, and underscores.
+
+## Basic Horizontal Rules
+
+Horizontal rules are created using three or more hyphens, asterisks, or underscores on their own line.
+
+---
+
+***
+
+___
+
+## Minimum Characters
+
+At least three characters are required, but more can be used.
+
+---
+
+----
+
+-----
+
+---
+
+## Different Characters
+
+Horizontal rules can be created with hyphens, asterisks, or underscores.
+
+---
+
+***
+
+___
+
+## Horizontal Rules in Context
+
+Horizontal rules can appear between paragraphs and other elements.
+
+This is a paragraph before the horizontal rule.
+
+---
+
+This is a paragraph after the horizontal rule.
+
+## Multiple Horizontal Rules
+
+Multiple horizontal rules can appear consecutively.
+
+---
+
+---
+
+---
+
+## Horizontal Rules with Formatting
+
+Horizontal rules are standalone elements and cannot contain formatting, but they can appear alongside formatted content.
+
+This paragraph has **bold text**.
+
+---
+
+This paragraph has *italic text*.
+
+## Horizontal Rules with Lists
+
+Horizontal rules can appear before and after lists.
+
+---
+
+- First item
+- Second item
+- Third item
+
+---
+
+## Horizontal Rules with Blockquotes
+
+Horizontal rules can appear before and after blockquotes.
+
+---
+
+> This is a blockquote.
+
+---
+
+## Horizontal Rules with Code Blocks
+
+Horizontal rules can appear before and after code blocks.
+
+---
+
+```
+Code block here
+```
+
+---
+
+## Horizontal Rules with Headings
+
+Horizontal rules can appear before and after headings.
+
+---
+
+# Heading Level 1
+
+---
+
+## Heading Level 2
+
+---
+
+## Spacing Around Horizontal Rules
+
+Horizontal rules should be on their own line with blank lines before and after for best compatibility.
+
+Paragraph before.
+
+---
+
+Paragraph after.
+
+## Edge Cases
+
+### Horizontal Rule with Spaces
+
+- - -
+
+* * *
+
+_ _ _
+
+### Horizontal Rule with Only Two Characters
+
+--
+
+**
+
+__
+
+### Horizontal Rule with Mixed Characters
+
+-*_
+
+*-_
+
+_*-
+
+### Horizontal Rule with Trailing Spaces
+
+---    
+
+***    
+
+___    
+
+### Horizontal Rule with Leading Spaces
+
+    ---
+
+    ***
+
+    ___
+
+### Very Long Horizontal Rules
+
+-----------------------------------
+
+***********************************
+
+___________________________________
+
+### Horizontal Rule Between Other Elements
+
+# Heading
+
+---
+
+## Another Heading
+
+---
+
+### Third Heading
+
+---
+
+Paragraph text.
+