refactor(cache)!: extract hybrid_cache package

[j2rong4cn]

Description / 描述

• 主要改动

  • 将 HybridCache 相关的代码移动到 internal/hybrid_cache 包中
  • 引入 BackingStore 抽象，包含 BufferStore 和 FileStore
  • 在 config/env/bootstrap 中将 CacheThreshold 重命名为 AutoMemoryLimit

• HybridCache 的缓存策略

  • 小于等于auto_memory_limit的数据流，仅使用[]byte缓存（Go自动管理内存），无回退文件缓存
  • 大于auto_memory_limit的数据流，优先使用手动管理内存（Unix Mmap或者Windows VirtualAlloc）用作缓存，内存不足回退到文件缓存
  • min_free_memory小于0时，仅使用文件缓存

• HybridCache 相关配置项：

  • auto_memory_limit 单位MB
    • 默认值：4
    • 在HybridCache中使用[]byte缓存数据流的限制，内存为自动管理，直到GC。
  • min_free_memory 单位MB
    • 默认值：小于16为自动大小，总内存的10%，最大1024
    • 最小空闲内存，当内存不足时，HybridCache会回退到文件缓存。
    • 如果小于0，HybridCache仅使用文件缓存，不占用内存。
  • max_block_limit 单位BM
    • 默认值：小于4为自动大小，总内存的3%，最大64
    • 限制HybridCache手动管理内存单次的扩容大小，超过该阈值将分多次扩容（为了检测内存是否充足）。 min_free_memory大于0时，也限制多线程下载的分片大小

Motivation and Context / 背景

Relate #2460

How Has This Been Tested? / 测试

• go test -run '^(TestRangeRead|TestPreHash|TestStreamSectionReader)$' ./internal/stream
• go test -run '^(TestDownloadOrder|TestHighConcurrency|TestDownloadSingle)$' ./internal/net
• go test ./internal/hybrid_cache

Checklist / 检查清单

• I have read the CONTRIBUTING document.
  我已阅读 CONTRIBUTING 文档。
• I have formatted my code with go fmt or prettier.
  我已使用 go fmt 或 prettier 格式化提交的代码。
• I have added appropriate labels to this PR (or mentioned needed labels in the description if lacking permissions).
  我已为此 PR 添加了适当的标签（如无权限或需要的标签不存在，请在描述中说明，管理员将后续处理）。
• I have requested review from relevant code authors using the "Request review" feature when applicable.
  我已在适当情况下使用"Request review"功能请求相关代码作者进行审查。
• I have updated the repository accordingly (If it’s needed).
  我已相应更新了相关仓库（若适用）。
  • OpenList-Frontend #XXXX
  • OpenList-Docs #XXXX

[Copilot]

Pull request overview

This PR refactors stream caching by extracting the former internal/mem.HybridCache into a dedicated internal/hybrid_cache package (with BackingStore abstractions), and introduces a breaking configuration rename from cache_threshold to auto_memory_limit to control when Go-managed memory vs manual/file-backed caching is used.

Changes:

• Extract HybridCache into internal/hybrid_cache and add BackingStore implementations (BufferStore, FileStore).
• Remove pkg/buffer’s old Reader/PeekFile utilities and consolidate NewByteBlock into a smaller file.
• Rename config cache_threshold → auto_memory_limit and update bootstrap + tests accordingly.

Reviewed changes

Copilot reviewed 19 out of 19 changed files in this pull request and generated 8 comments.

Show a summary per file

File	Description
pkg/buffer/utils.go	Moves provider interface types out; keeps adapter helpers.
pkg/buffer/type.go	Defines WriteAtSeekerProvider / ReadAtSeekerProvider alongside core buffer types.
pkg/buffer/file.go	Removes PeekFile implementation (deleted).
pkg/buffer/bytes.go	Removes buffer.Reader and embedded byteBlock (deleted).
pkg/buffer/buffer.go	Reintroduces NewByteBlock + byteBlock in a dedicated file.
internal/stream/util.go	Switches stream section caching to internal/hybrid_cache.
internal/stream/stream.go	Reworks stream caching to always use the new HybridCache path.
internal/stream/stream_test.go	Updates tests for the AutoMemoryLimit rename/behavior.
internal/net/request.go	Switches downloader caching to internal/hybrid_cache.
internal/mem/cache.go	Removes old HybridCache implementation (deleted).
internal/hybrid_cache/type.go	Introduces BackingStore interface.
internal/hybrid_cache/hybrid_cache.go	New HybridCache implementation and policy selection logic.
internal/hybrid_cache/file.go	File-backed store implementation and constructors.
internal/hybrid_cache/file_test.go	Updates tests to new package/types (MultiFileStore, GrowTo).
internal/hybrid_cache/buffer.go	Adds BufferStore (Go-managed memory backing store).
internal/hybrid_cache/buffer_test.go	Adds/updates tests for BufferStore.
internal/conf/var.go	Renames threshold var to AutoMemoryLimit and updates comments.
internal/conf/config.go	Renames config field/tag to auto_memory_limit and updates defaults.
internal/bootstrap/config.go	Reads and logs AutoMemoryLimit instead of CacheThreshold.

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