deprecate(CodeSnippet): Mark class as deprecated for removal in next major

[usernane]

Description

CodeSnippet is an opinionated UI component with hardcoded inline styles, a fragile line-counting mechanism, and a specific visual layout. It does not belong in a low-level DOM library.

Problems:

• Hardcodes inline CSS (floats, borders, colors)
• Line number counting has bugs (accumulates on repeated setCode() calls, off-by-one for empty input)
• No syntax highlighting — just plain text with numbers
• Any real project uses highlight.js, Prism, or Shiki instead
• Couples presentation decisions to infrastructure

Replacement (3 lines):

$pre = new HTMLNode('pre');
$code = $pre->addChild('code', ['class' => 'language-php']);
$code->text($sourceCode);

Proposed Change

• Mark CodeSnippet class with @deprecated annotation
• Mark HTMLNode::codeSnippet() shortcut method as @deprecated
• Remove both in next major version

Acceptance Criteria

• @deprecated tag added to class and shortcut method
• No functional changes (still works for existing users)
• PHPDoc points to <pre><code> pattern as replacement