perf: using writer to calculate concat source buffer

[SyMind]

This PR optimizes buffer calculation by replacing direct buffer concatenation with a writer-based approach to improve performance when calculating source buffers.

[codspeed-hq]

CodSpeed Performance Report

Merging #165 will not alter performance

_{Comparing SyMind:refactor-buffer (7ce7075) with main (ed6fa44)}

Summary

✅ 7 untouched

[Copilot]

Pull Request Overview

This PR optimizes buffer calculation by replacing direct buffer concatenation with a writer-based approach to improve performance when calculating source buffers.

• Replaces iterator-based buffer concatenation with writer approach in ConcatSource::buffer()
• Updates CachedSource::buffer() to use the same writer-based pattern for consistency

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/concat_source.rs	Replaces buffer concatenation logic with writer-based approach
src/cached_source.rs	Updates buffer method to use writer pattern instead of delegating to inner source

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