Support multi-line `text-overflow` behavior

[FlorianRappl]

Description
text-overflow behavior for multi-line scenarios is currently unsupported/incomplete.

Scope

• Implement overflow signaling behavior for multi-line text contexts as supported by renderer model.
• Handle ellipsis/truncation behavior consistently with line breaking and clipping.
• Define and document any intentional limitations.

Acceptance criteria

• Multi-line overflow scenarios produce stable, testable truncation behavior.
• Ellipsis behavior is applied where supported by current engine capabilities.
• Tests include varying widths, line heights, and nested inline content.