add recursion depth limit to prevent stack overflow

[wtn]

Prevents stack overflow DoS attacks by limiting recursion depth when parsing binary plists.

Changes

• Add MAX_DEPTH constant (512 levels)
• Thread depth parameter through recursive parsing methods
• Raise CFFormatError when depth exceeds limit
• Add tests for deeply nested arrays and dictionaries

Background

The binary parser had no recursion depth limiting, allowing attackers to craft deeply nested structures (2000+ levels) that would cause SystemStackError crashes.