feat: Complete Issue #41 - Add TOML frontmatter support

- Enhanced frontmatter parser to detect and parse TOML format
- Added TOML format detection heuristics before YAML parsing
- Created TOML test fixture with nested sections
- Fixed parsing order to prevent TOML-to-string conversion
- All frontmatter formats (YAML, JSON, TOML) now fully supported
- Validated all acceptance criteria for Issue #41

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/fixtures/frontmatter_test_files/toml_frontmatter.md

@@ -0,0 +1,29 @@
+---
+title = "TOML Frontmatter Test Document"
+author = "Test Author"
+date = "2025-10-02"
+tags = ["toml", "frontmatter", "testing"]
+version = 1.4
+published = true
+
+[config]
+theme = "light"
+language = "en"
+debug = false
+
+[features]
+search = true
+toc = true
+navigation = true
+---
+
+# TOML Frontmatter Test Document
+
+This document uses TOML format for frontmatter instead of YAML or JSON.
+
+The frontmatter parser should handle TOML format correctly and extract values like:
+- title: "TOML Frontmatter Test Document"
+- config.theme: "light"
+- features.search: true
+
+This tests the parser's ability to handle all three frontmatter formats as required by Issue #41.