chore: add Nix Flake support for reproducible builds and development

[wjkoh]

Hi SQLite AI team,

First off, thank you for open-sourcing sqlite-vector. It is a fantastic extension, and the focus on "zero preindexing" and low memory footprint makes it incredibly useful for edge applications.

I’ve put together a PR that adds Nix Flake support to the repository.

Why this is useful

Nix provides a purely functional deployment model that ensures reproducible builds across different environments. By adding a flake.nix, we allow developers and users to:

1. Onboard instantly: Run nix develop to drop into a shell with all dependencies (sqlite, make, compilers) pre-configured. No need to manually install system libraries or worry about version mismatches.
2. Build and Install effortlessly: Users can build and install the extension directly using the Nix package manager (e.g., nix build github:sqliteai/sqlite-vector), or easily include it as a dependency in their own Nix-based projects.
3. Build reproducibly: Run nix build to compile the extension in a sandboxed environment, ensuring it works strictly with the declared dependencies.
4. Test easily: The development shell automatically exports a SQLITE_VECTOR_LIB environment variable pointing to the built extension, making it trivial to test:

   # In the nix develop shell
   sqlite3 :memory: ".load $SQLITE_VECTOR_LIB" "SELECT vector_version();"

What changed

This PR adds two files: flake.nix and flake.lock.

• It utilizes the existing Makefile workflow, so it does not interfere with your current build process.
• It handles platform-specific quirks automatically (e.g., correctly setting CC and handling macOS vs. Linux shared library extensions).

I hope you find this addition helpful for the community!

[marcobambini]

Hi @wjkoh,

Thanks very much for your pull request that I've just accepted and merged.
Please let me know if there's anything we can do to improve sqlite-vector!

[wjkoh]

Thanks @marcobambini for the swiftest PR merge I've ever seen! I wrote a blog post about it: https://cwrk.ai/posts/sqlite-vector-nix-flake-support/. This change will save a lot of dev hours. Thanks again for making this happen!

[marcobambini]

Thanks a lot @wjkoh for the blog post!
I really appreciate it!