fix: prevent extra blank lines after headings with hard breaks

When a heading ends with a hard break (\), the content continues on the
next line, so we should not add an extra blank line. This fixes the bug
where comment lines ending with hard breaks would have blank lines
incorrectly inserted after them.

Also added comprehensive test suite for heading spacing and hard line
breaks to prevent regressions.

Changes:
- Modified render_heading to detect headings ending with backslash
- When hard break detected, use single newline instead of double
- Added test_heading_spacing.py with 9 tests covering:
  - Basic heading spacing (blank line after headings)
  - Headings before lists, quotes, and code blocks
  - Headings with hard breaks (the corner case)
  - Hard breaks in regular paragraphs and list items
  - Hard breaks after comment lines

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

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

test: add bold paragraph before list test case

Added test case with bold text paragraph followed by list to
verify blank line insertion works correctly.

Example:
  **Related Architecture**:
  - [link] - description

Output correctly adds blank line after the bold paragraph.

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

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

Test on Python 3.14.

Copier update.

Update readme.

Small improvement to smartquotes.

Small smartquote improvement.

Add --width option.

Auto-format readme.

Bugfix of title quotes.