refactor: simply the code

[hustcc]

• refactor: simplify the code

[gemini-code-assist]

Code Review

This pull request refactors and simplifies the codebase by consolidating the embedding, query expansion, and storage layers, notably replacing EmbedderManager with a unified Embedder class and migrating SynonymExpander to a pure expand function. The review highlights several critical issues to address, including a regression in file loading where switching to Promise.all makes the process fragile to single-file failures, a potential crash in query options destructuring when topK is explicitly undefined, a race condition in the static getPipeline initialization, a typo in the CJK regex range, and a lack of runtime validation for parsed metadata.

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