feat: add text overflow detection to TextFrame

[saggl]

Summary

Adds will_overflow() and overflow_info() methods to TextFrame for detecting text overflow before rendering. Supports vertical, horizontal, and combined detection.

API

# Simple check — does text overflow?
text_frame.will_overflow(font_size=18)  # -> bool

# Detailed metrics
info = text_frame.overflow_info(font_size=18)
info.overflow_percentage   # how much it overflows
info.fits_at_font_size     # font size that would fit

# Horizontal overflow (URLs, long words that break word-wrap)
text_frame.will_overflow(direction='horizontal', font_size=18)

# Check both directions at once
text_frame.will_overflow(direction='both', font_size=18)

will_overflow(direction, font_family, font_size, bold, italic, font_file) -> bool
overflow_info(...) -> OverflowInfo — returns dimensional metrics, overflow percentages, problematic elements, and suggested font sizes.

Default direction='vertical' preserves backward compatibility.

Changes

• src/pptx/text/text.py — OverflowInfo frozen dataclass, will_overflow(), overflow_info() on TextFrame
• src/pptx/text/layout.py — TextFitter methods: will_fit, will_fit_width, measure_text_height, measure_text_width, calculate_overflow_metrics, calculate_horizontal_overflow_metrics; empty-string guard in _rendered_size
• tests/ — unit tests for all new methods and edge cases (vertical, horizontal, both, invalid direction, empty text, mixed fonts)
• features/ — 12 BDD scenarios covering vertical, horizontal, and error paths
• examples/ — usage examples for both overflow directions

Test plan

• pytest tests/ -x — 2723 tests pass
• behave features/txt-overflow-detection.feature — 12 scenarios pass
• Backward compatible: default direction='vertical' unchanged