Add a dev-overlay nix flake

Based on the impure template from `pyproject.nix` and providing
a dev-shell for easy bypass-n-hack on nix(os) using `uv`.

Deats,
- include bash completion pkgs for devx/happiness.
- pull `ruff` from <nixpkgs> to avoid wheel (build) issues.
- pin to py313 `cpython` for now.

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1769018530,
+        "narHash": "sha256-MJ27Cy2NtBEV5tsK+YraYr2g851f3Fl1LpNHDzDX15c=",
+        "owner": "nixos",
+        "repo": "nixpkgs",
+        "rev": "88d3861acdd3d2f0e361767018218e51810df8a1",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nixos",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,70 @@
+# An "impure" template thx to `pyproject.nix`,
+# https://pyproject-nix.github.io/pyproject.nix/templates.html#impure
+# https://github.com/pyproject-nix/pyproject.nix/blob/master/templates/impure/flake.nix
+{
+  description = "An impure overlay using `uv` with Nix(OS)";
+
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    { nixpkgs, ... }:
+    let
+      inherit (nixpkgs) lib;
+      forAllSystems = lib.genAttrs lib.systems.flakeExposed;
+    in
+    {
+      devShells = forAllSystems (
+        system:
+        let
+          pkgs = nixpkgs.legacyPackages.${system};
+
+          # XXX NOTE XXX, for now we overlay specific pkgs via
+          # a major-version-pinned-`cpython`
+          cpython = "python313";
+          venv_dir = "py313";
+          pypkgs = pkgs."${cpython}Packages";
+        in
+        {
+          default = pkgs.mkShell {
+
+            packages = with pkgs; [
+              # XXX, ensure sh completions activate!
+              bashInteractive
+              bash-completion
+
+              # on nixos, use pkg(s)
+              ruff
+              pypkgs.ruff
+
+              uv
+              python313  # ?TODO^ how to set from `cpython` above?
+            ];
+
+            shellHook = ''
+              # unmask to debug **this** dev-shell-hook
+              # set -e
+
+              # link-in c++ stdlib for various AOT-ext-pkgs (numpy, etc.)
+              LD_LIBRARY_PATH="${pkgs.stdenv.cc.cc.lib}/lib:$LD_LIBRARY_PATH"
+
+              export LD_LIBRARY_PATH
+
+              # RUNTIME-SETTINGS
+              # ------ uv ------
+              # - always use the ./py313/ venv-subdir
+              # - sync env with all extras
+              export UV_PROJECT_ENVIRONMENT=${venv_dir}
+              uv sync --dev --all-extras --no-group lint
+
+              # ------ TIPS ------
+              # NOTE, to launch the py-venv installed `xonsh` (like @goodboy)
+              # run the `nix develop` cmd with,
+              # >> nix develop -c uv run xonsh
+            '';
+          };
+        }
+      );
+    };
+}