feat: add text overflow detection to TextFrame

[saggl]

Add new methods to detect and analyze text overflow before it occurs:

• TextFrame.will_overflow(): Simple boolean check for overflow
• TextFrame.overflow_info(): Detailed overflow metrics with OverflowInfo dataclass
• TextFitter.will_fit(): Core overflow detection logic
• TextFitter.measure_text_height(): Calculate required text height
• TextFitter.calculate_overflow_metrics(): Detailed overflow analysis

The OverflowInfo dataclass provides:

• will_overflow: Boolean indicator
• required_height/available_height: Size metrics
• overflow_height/overflow_percentage: Overflow amounts
• estimated_lines: Line count estimate
• fits_at_font_size: Recommended font size to fit

This enables proactive content validation to prevent text from being cut off at the bottom of text frames, addressing the issue where users don't know content will overflow until they open the presentation.

Includes comprehensive unit tests, feature tests, and examples.