fix(toc): more clean toc rendering

[xihale]

This PR improves Table of Contents (TOC) generation to correctly handle documents that start with H2 (or lower) headers, and optimizes the parsing logic.

Close: #186

Changes

src/render/html.zig

• Auto-detect Root Level: The TOC generator now scans for the minimum heading level (min_level) in the document and uses that as the root.
• No Output for Empty TOC: Returns early if no headings are found, preventing empty <ul></ul> tags.

Tests

• Added tests/rendering/toc_bug/ reproduction case.
  • Input: index.smd with only H2 headers.
  • Output: snapshot/toc/index.html confirms a flat <ul> list without nested empty items.

Breaking

Before, detecting whether a toc is empty we use $page.toc().len().gt(10), but now we use $page.toc().len().gt(0).