fix(flake): build and install the rust package

- Add crane as a flake input for idiomatic Rust builds
- Configure package build with proper dependencies (openssl, pkg-config, perl)
- Add macOS-specific frameworks for Darwin compatibility
- Export package as default and named output
- Add app definition for `nix run` support
- Update Cargo.lock for dependency compatibility

The flake now properly builds the rust-docs-mcp-server binary and makes it
available via `nix build`, `nix run`, or `nix profile install`.

Author: brndnmtthws