Refactor/buf doc loading

[kibi2]

Summary

This PR introduces a lightweight buffer state flag to distinguish between flat text buffers and tir-buf buffers.

Previously, the buffer type could be inferred by scanning the entire buffer content and checking whether it contained pipe (|) characters. While this approach worked, it required reading the full buffer repeatedly and introduced unnecessary overhead.

To avoid that cost, this PR stores the buffer state (FLAT) as a buffer-local flag instead.

In addition, this PR includes some internal cleanup:

• renamed several functions for clarity
• removed unused functions
• updated and expanded test cases

Notes

The new flag slightly increases internal state management complexity, but should improve performance and simplify buffer-type checks in hot paths.

[codecov]

Codecov Report

❌ Patch coverage is 95.95376% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lua/tirenvi/core/dirty_range.lua	88.88%	4 Missing ⚠️
lua/tirenvi/app/pipeline.lua	97.22%	1 Missing ⚠️
lua/tirenvi/core/attrs.lua	75.00%	1 Missing ⚠️
lua/tirenvi/editor/autocmd.lua	95.65%	1 Missing ⚠️

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