Introduce BufferOverflowLine rows that share cell storage with a parent
BufferLine head, implementing the logical-line model from issue #4800.
- Extend BufferLine with logical capacity and segment-aware indexing
- Add BufferOverflowLine, BufferLineUtils helpers, and unit tests
- Expose attachWrappedRow/clearWrappedRow on Buffer for future integration
- Keep InputHandler on isWrapped flag until reflow/SL paths are updated
Reflow still uses per-row buffers; materializeLogicalLinesForReflow helpers
are included for a follow-up that simplifies resize reflow.
Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>
