Feat/line provider

[kibi2]

Summary

Introduce a LineProvider abstraction and improve multiline cell rendering.

Changes

• Add LineProvider to decouple line access logic
• Implement multiline cell rendering (clean version)
• Use buffer cache to efficiently detect block boundaries

Notes

This refactor prepares for future improvements in rendering and block handling.
No breaking changes are expected.

[codecov]

Codecov Report

❌ Patch coverage is 95.38462% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lua/tirenvi/core/tir_vim.lua	96.15%	1 Missing ⚠️
lua/tirenvi/state/buffer.lua	66.66%	1 Missing ⚠️
lua/tirenvi/state/buffer_line_provider.lua	87.50%	1 Missing ⚠️

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