refactor: extract cursor handling into a module

[kibi2]

Summary

Refactor cursor handling by extracting cursor-related logic into a dedicated module.

Changes

• Move cursor position handling into a cursor module
• Centralize cursor state management
• Separate cursor information (buffer position, logical position, block/cell location) from other editor logic
• Prepare the code structure for future cursor restoration and movement improvements

Motivation

Cursor handling has become spread across multiple parts of Tirenvi as logical table positions and rendered buffer positions need to be synchronized.

This refactoring makes cursor-related state and operations easier to maintain and provides a clearer foundation for future features such as logical cursor restoration and structured table editing improvements.

Testing

• Existing test suite passes
• Verified cursor movement and restoration behavior

[codecov]

Codecov Report

❌ Patch coverage is 97.26562% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lua/tirenvi/width/op.lua	66.66%	4 Missing ⚠️
lua/tirenvi/core/dirty_range.lua	83.33%	1 Missing ⚠️
lua/tirenvi/editor/autocmd.lua	83.33%	1 Missing ⚠️
lua/tirenvi/io/buffer.lua	95.00%	1 Missing ⚠️

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