Feat/multiline cell lf

[kibi2]

Summary

This PR introduces multiline cell rendering in tirenvi.

Cells containing line breaks (\n) are now rendered across multiple rows, enabling more natural and readable table editing.

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Key Changes

• Support rendering a single cell as multiple visual rows when it contains LF (\n)
• Introduce continuation handling between rows
• Add a lightweight buffer line cache for efficient upward/downward traversal
• Update rendering behavior (including pipe and conceal handling) to support multiline layout

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Design Notes

• Multiline rendering is treated as a core rendering model, not an optional feature

• Each visual row is internally linked using a continuation flag

• Buffer access is optimized via a single-chunk cache tailored for directional scanning

• This implementation serves as a foundation for upcoming features, such as:

  • column-width-based wrapping
  • fold support for multiline cells

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Scope

This PR focuses on:

• LF-based multiline rendering

It does not yet include:

• column width wrapping (planned next)
• fold integration (planned)

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Testing

• Added/updated test cases for buffer access and cache behavior

• Verified correct behavior for:

  • multiline cell rendering
  • continuation handling
  • cache-based line access

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Notes

This PR restructures part of the rendering pipeline, so reviewing the overall flow (rather than individual diffs) may be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 95.43726% with 12 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lua/tirenvi/core/record.lua	89.28%	6 Missing ⚠️
lua/tirenvi/state/buffer.lua	94.82%	3 Missing ⚠️
lua/tirenvi/core/tir_vim.lua	97.61%	1 Missing ⚠️
lua/tirenvi/ui.lua	96.96%	1 Missing ⚠️
lua/tirenvi/util/log.lua	87.50%	1 Missing ⚠️

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