feat: support nix flake and minimize rust binary size

[thinkgos]

I use it on Nix, but the upstream repository doesn't support it yet. I've added Nix flake support in this PR and using it in my home-manager configuration.

Additionally, minimize rust binary size.

Summary by Sourcery

Add Nix flake support and tune the Rust release build profile for smaller binaries.

New Features:

• Provide a Nix flake definition to build and package the project across common Linux and macOS systems.
• Add a Nix development shell configuration with an appropriate Rust toolchain and environment setup.

Enhancements:

• Configure the Rust release profile to optimize for minimal binary size via more aggressive compilation settings.

[sourcery-ai]

Reviewer's Guide

Adds Nix flake-based packaging and development environment, and tightens Rust release build settings to minimize the compiled binary size.

File-Level Changes

Change	Details	Files
Configure Rust release profile for smaller, more optimized binaries.	Set release opt-level to 'z' to optimize for binary size over speed Enable symbol stripping in release builds Enable Link Time Optimization (LTO) for release builds Reduce codegen-units to 1 for better cross-crate optimization at the cost of compile time Switch panic strategy to 'abort' in release to remove unwinding support and shrink binaries	Cargo.toml
Introduce Nix flake for building, packaging, and developing the project on Nix-based systems.	Define flake inputs for nixpkgs, flake-parts, and oxalica rust-overlay Declare supported systems matrix for Linux and Darwin on x86_64 and aarch64 Configure pkgs import to use rust-overlay via flake-parts perSystem Create a Rust build environment using makeRustPlatform and rust-bin toolchains Derive package name and version from Cargo.toml via fromTOML Define a default package using buildRustPackage with Cargo.lock and non-auditable build Set package metadata including homepage, description, license, and mainProgram for Nix consumers Provide a devShell that reuses package dependencies and installs a full Rust toolchain with rust-analyzer, setting RUST_SRC_PATH and a shell greeting	flake.nix flake.lock

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[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• The [profile.release] changes (especially panic = "abort", lto = true, and codegen-units = 1) globally affect all release builds and can make debugging and integration with other tooling harder; consider moving these to an opt-in profile or gating them behind a Cargo feature to avoid surprising consumers.
• In flake.nix, you define both rustToolchain and a separate rustPlatform using rust-bin.stable.latest.minimal; you could simplify by deriving rustPlatform from rustToolchain (or vice versa) to avoid duplicating version/toolchain configuration.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `[profile.release]` changes (especially `panic = "abort"`, `lto = true`, and `codegen-units = 1`) globally affect all release builds and can make debugging and integration with other tooling harder; consider moving these to an opt-in profile or gating them behind a Cargo feature to avoid surprising consumers.
- In `flake.nix`, you define both `rustToolchain` and a separate `rustPlatform` using `rust-bin.stable.latest.minimal`; you could simplify by deriving `rustPlatform` from `rustToolchain` (or vice versa) to avoid duplicating version/toolchain configuration.

## Individual Comments

### Comment 1
<location path="flake.nix" line_range="62-53" />
<code_context>
+        {
+          # overlay
+          # https://flake.parts/overlays.html#consuming-an-overlay
+          _module.args.pkgs = import inputs.nixpkgs {
+            inherit system;
+            overlays = [ (import inputs.rust-overlay) ];
</code_context>
<issue_to_address>
**issue (bug_risk):** The `pkgs` used in `let` bindings likely doesn't include the `rust-overlay`, which can break `pkgs.rust-bin.*` and `makeRustPlatform`.

In this `perSystem` block, the `pkgs` in the `let` come from the un-overlaid argument and are only later replaced via `_module.args.pkgs = import inputs.nixpkgs { ... overlays = [ (import inputs.rust-overlay) ]; };`. As a result, `rustToolchain` / `rustPlatform` are likely defined from a `pkgs` without `rust-overlay`, so `pkgs.rust-bin.*` may be missing or inconsistent.

To avoid this ordering issue, import `nixpkgs` with the overlay into a dedicated binding and use it everywhere, for example:

```nix
let
  pkgsWithRust = import inputs.nixpkgs {
    inherit system;
    overlays = [ (import inputs.rust-overlay) ];
  };
  rustToolchain = pkgsWithRust.rust-bin.stable.latest.default.override { ... };
  rustPlatform = pkgsWithRust.makeRustPlatform { ... };
  cargoToml = fromTOML (builtins.readFile ./Cargo.toml);
in {
  _module.args.pkgs = pkgsWithRust;
  ...
}
```

This keeps the overlayed `pkgs` consistent and avoids subtle breakage if the `pkgs` argument changes.
</issue_to_address>

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