feat(buffer): logical line storage for wrapped rows

[Tyriar]

Summary

Implements the logical-line model described in #4800 for wrapped buffer rows: continuation rows share the parent BufferLine cell storage instead of duplicating data and relying on isWrapped on each physical row.

Changes

• BufferLine: supports extended logical capacity (logicalCellCount) and segment-aware cell indexing via _segmentStart
• BufferOverflowLine: viewport row that views a slice of the parent line's shared _data / attribute maps
• BufferLineUtils: helpers to attach/clear wrapped rows, flatten logical groups for reflow, and create scroll-wrapped lines
• Buffer API: attachWrappedRow, clearWrappedRow, createScrollWrappedLine
• InputHandler: _unwrapBufferRow handles logical overflow rows when clearing wrap (e.g. backspace, erase); print path still uses isWrapped until reflow/SL integration is completed
• Tests: BufferLogicalLine.test.ts covers shared storage across wrapped rows

Follow-up (not in this PR)

• Wire attachWrappedRow into the InputHandler print/wrap hot path
• Use materializeLogicalLinesForReflow before buffer reflow to simplify _reflowSmaller / _reflowLarger (avoid copying cells between rows that already share storage)
• Optional: enable logical scroll-wrapped lines in BufferService.scroll

Testing

• npm run test-unit -- out-esbuild/common/buffer/ out-esbuild/common/InputHandler.test.js (316 passing)
• npm run lint-changes

  

[PerBothner]

It's late, and I haven't had a chance to understand this change. However, I should point out that I believe that PR #5797 is a cleaner and more refined variation of the ideas in issue #4800: Instead of dealing with "overflow lines" it explicitly uses a LogicalLine class. PR #5853 builds on PR #5797 in also re-implementing Markers.

[Tyriar]

@PerBothner this is just me experimenting with a throw away agent implementation, I didn't want it to make the PR (still figuring out how to do that).