perf: should flatten nested concat sources

[SyMind]

This PR implements flattening for nested ConcatSource instances to improve performance by avoiding deeply nested source structures. The optimization flattens ConcatSource children during construction and addition operations.

[Copilot]

Pull Request Overview

This PR implements flattening for nested ConcatSource instances to improve performance by avoiding deeply nested source structures. The optimization flattens ConcatSource children during construction and addition operations.

• Refactored ConcatSource::new() to use iterative add() method instead of flat_map
• Enhanced add() method to flatten both direct ConcatSource instances and BoxSource-wrapped ConcatSource instances
• Updated buffer() implementation to use to_writer() for consistency

Reviewed Changes

Copilot reviewed 1 out of 1 changed files in this pull request and generated 3 comments.

File	Description
src/concat_source.rs	Main implementation of nested ConcatSource flattening with updated constructor, add method, and buffer implementation
src/cached_source.rs	Updated buffer method to use to_writer for consistency with ConcatSource changes

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[codspeed-hq]

CodSpeed Performance Report

Merging #166 will not alter performance

_{Comparing SyMind:refactor-buffer (1a2fc63) with main (0ac0616)}

Summary

✅ 7 untouched