diff --git a/flake.lock b/flake.lock
new file mode 100644
index 00000000..5c269493
--- /dev/null
+++ b/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
+}
diff --git a/flake.nix b/flake.nix
new file mode 100644
index 00000000..b46ef43c
--- /dev/null
+++ b/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 (w dev-shell) using `uv`";
+
+  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 = [
+              # XXX, ensure sh completions activate!
+              pkgs.bashInteractive
+              pkgs.bash-completion
+
+              # XXX, on nix(os), use pkgs version to avoid
+              # build/sys-sh-integration issues
+              pkgs.ruff
+
+              pkgs.uv
+              pkgs.${cpython}# ?TODO^ how to set from `cpython` above?
+            ];
+
+            shellHook = ''
+              # unmask to debug **this** dev-shell-hook
+              # set -e
+
+              # link-in c++ stdlib for various AOT-ext-pkgs (numpy, etc.)
+              LD_LIBRARY_PATH="${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
+
+              # ------ TIPS ------
+              # NOTE, to launch the py-venv installed `xonsh` (like @goodboy)
+              # run the `nix develop` cmd with,
+              # >> nix develop -c uv run xonsh
+            '';
+          };
+        }
+      );
+    };
+}
diff --git a/pyproject.toml b/pyproject.toml
index a0491598..d3a344e1 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -53,22 +53,33 @@ dependencies = [
 
 [dependency-groups]
 dev = [
-  # test suite
-  # TODO: maybe some of these layout choices?
-  # https://docs.pytest.org/en/8.0.x/explanation/goodpractices.html#choosing-a-test-layout-import-rules
-  "pytest>=8.3.5",
-  "pexpect>=4.9.0,<5",
+  {include-group = 'devx'},
+  {include-group = 'testing'},
+  {include-group = 'repl'},
+]
+devx = [
   # `tractor.devx` tooling
   "greenback>=1.2.1,<2",
   "stackscope>=0.2.2,<0.3",
   # ^ requires this?
   "typing-extensions>=4.14.1",
-
+]
+testing = [
+  # test suite
+  # TODO: maybe some of these layout choices?
+  # https://docs.pytest.org/en/8.0.x/explanation/goodpractices.html#choosing-a-test-layout-import-rules
+  "pytest>=8.3.5",
+  "pexpect>=4.9.0,<5",
+]
+repl = [
   "pyperclip>=1.9.0",
   "prompt-toolkit>=3.0.50",
   "xonsh>=0.19.2",
   "psutil>=7.0.0",
 ]
+lint = [
+  "ruff>=0.9.6"
+]
 # TODO, add these with sane versions; were originally in
 # `requirements-docs.txt`..
 # docs = [
diff --git a/uv.lock b/uv.lock
index 3c05dc2f..3e94f1cf 100644
--- a/uv.lock
+++ b/uv.lock
@@ -1,5 +1,5 @@
 version = 1
-revision = 2
+revision = 3
 requires-python = ">=3.11"
 
 [[package]]
@@ -329,6 +329,32 @@ wheels = [
     { url = "https://files.pythonhosted.org/packages/30/3d/64ad57c803f1fa1e963a7946b6e0fea4a70df53c1a7fed304586539c2bac/pytest-8.3.5-py3-none-any.whl", hash = "sha256:c69214aa47deac29fad6c2a4f590b9c4a9fdb16a403176fe154b79c0b4d4d820", size = 343634, upload-time = "2025-03-02T12:54:52.069Z" },
 ]
 
+[[package]]
+name = "ruff"
+version = "0.14.14"
+source = { registry = "https://pypi.org/simple" }
+sdist = { url = "https://files.pythonhosted.org/packages/2e/06/f71e3a86b2df0dfa2d2f72195941cd09b44f87711cb7fa5193732cb9a5fc/ruff-0.14.14.tar.gz", hash = "sha256:2d0f819c9a90205f3a867dbbd0be083bee9912e170fd7d9704cc8ae45824896b", size = 4515732, upload-time = "2026-01-22T22:30:17.527Z" }
+wheels = [
+    { url = "https://files.pythonhosted.org/packages/d2/89/20a12e97bc6b9f9f68343952da08a8099c57237aef953a56b82711d55edd/ruff-0.14.14-py3-none-linux_armv6l.whl", hash = "sha256:7cfe36b56e8489dee8fbc777c61959f60ec0f1f11817e8f2415f429552846aed", size = 10467650, upload-time = "2026-01-22T22:30:08.578Z" },
+    { url = "https://files.pythonhosted.org/packages/a3/b1/c5de3fd2d5a831fcae21beda5e3589c0ba67eec8202e992388e4b17a6040/ruff-0.14.14-py3-none-macosx_10_12_x86_64.whl", hash = "sha256:6006a0082336e7920b9573ef8a7f52eec837add1265cc74e04ea8a4368cd704c", size = 10883245, upload-time = "2026-01-22T22:30:04.155Z" },
+    { url = "https://files.pythonhosted.org/packages/b8/7c/3c1db59a10e7490f8f6f8559d1db8636cbb13dccebf18686f4e3c9d7c772/ruff-0.14.14-py3-none-macosx_11_0_arm64.whl", hash = "sha256:026c1d25996818f0bf498636686199d9bd0d9d6341c9c2c3b62e2a0198b758de", size = 10231273, upload-time = "2026-01-22T22:30:34.642Z" },
+    { url = "https://files.pythonhosted.org/packages/a1/6e/5e0e0d9674be0f8581d1f5e0f0a04761203affce3232c1a1189d0e3b4dad/ruff-0.14.14-py3-none-manylinux_2_17_aarch64.manylinux2014_aarch64.whl", hash = "sha256:f666445819d31210b71e0a6d1c01e24447a20b85458eea25a25fe8142210ae0e", size = 10585753, upload-time = "2026-01-22T22:30:31.781Z" },
+    { url = "https://files.pythonhosted.org/packages/23/09/754ab09f46ff1884d422dc26d59ba18b4e5d355be147721bb2518aa2a014/ruff-0.14.14-py3-none-manylinux_2_17_armv7l.manylinux2014_armv7l.whl", hash = "sha256:3c0f18b922c6d2ff9a5e6c3ee16259adc513ca775bcf82c67ebab7cbd9da5bc8", size = 10286052, upload-time = "2026-01-22T22:30:24.827Z" },
+    { url = "https://files.pythonhosted.org/packages/c8/cc/e71f88dd2a12afb5f50733851729d6b571a7c3a35bfdb16c3035132675a0/ruff-0.14.14-py3-none-manylinux_2_17_i686.manylinux2014_i686.whl", hash = "sha256:1629e67489c2dea43e8658c3dba659edbfd87361624b4040d1df04c9740ae906", size = 11043637, upload-time = "2026-01-22T22:30:13.239Z" },
+    { url = "https://files.pythonhosted.org/packages/67/b2/397245026352494497dac935d7f00f1468c03a23a0c5db6ad8fc49ca3fb2/ruff-0.14.14-py3-none-manylinux_2_17_ppc64.manylinux2014_ppc64.whl", hash = "sha256:27493a2131ea0f899057d49d303e4292b2cae2bb57253c1ed1f256fbcd1da480", size = 12194761, upload-time = "2026-01-22T22:30:22.542Z" },
+    { url = "https://files.pythonhosted.org/packages/5b/06/06ef271459f778323112c51b7587ce85230785cd64e91772034ddb88f200/ruff-0.14.14-py3-none-manylinux_2_17_ppc64le.manylinux2014_ppc64le.whl", hash = "sha256:01ff589aab3f5b539e35db38425da31a57521efd1e4ad1ae08fc34dbe30bd7df", size = 12005701, upload-time = "2026-01-22T22:30:20.499Z" },
+    { url = "https://files.pythonhosted.org/packages/41/d6/99364514541cf811ccc5ac44362f88df66373e9fec1b9d1c4cc830593fe7/ruff-0.14.14-py3-none-manylinux_2_17_s390x.manylinux2014_s390x.whl", hash = "sha256:1cc12d74eef0f29f51775f5b755913eb523546b88e2d733e1d701fe65144e89b", size = 11282455, upload-time = "2026-01-22T22:29:59.679Z" },
+    { url = "https://files.pythonhosted.org/packages/ca/71/37daa46f89475f8582b7762ecd2722492df26421714a33e72ccc9a84d7a5/ruff-0.14.14-py3-none-manylinux_2_17_x86_64.manylinux2014_x86_64.whl", hash = "sha256:bb8481604b7a9e75eff53772496201690ce2687067e038b3cc31aaf16aa0b974", size = 11215882, upload-time = "2026-01-22T22:29:57.032Z" },
+    { url = "https://files.pythonhosted.org/packages/2c/10/a31f86169ec91c0705e618443ee74ede0bdd94da0a57b28e72db68b2dbac/ruff-0.14.14-py3-none-manylinux_2_31_riscv64.whl", hash = "sha256:14649acb1cf7b5d2d283ebd2f58d56b75836ed8c6f329664fa91cdea19e76e66", size = 11180549, upload-time = "2026-01-22T22:30:27.175Z" },
+    { url = "https://files.pythonhosted.org/packages/fd/1e/c723f20536b5163adf79bdd10c5f093414293cdf567eed9bdb7b83940f3f/ruff-0.14.14-py3-none-musllinux_1_2_aarch64.whl", hash = "sha256:e8058d2145566510790eab4e2fad186002e288dec5e0d343a92fe7b0bc1b3e13", size = 10543416, upload-time = "2026-01-22T22:30:01.964Z" },
+    { url = "https://files.pythonhosted.org/packages/3e/34/8a84cea7e42c2d94ba5bde1d7a4fae164d6318f13f933d92da6d7c2041ff/ruff-0.14.14-py3-none-musllinux_1_2_armv7l.whl", hash = "sha256:e651e977a79e4c758eb807f0481d673a67ffe53cfa92209781dfa3a996cf8412", size = 10285491, upload-time = "2026-01-22T22:30:29.51Z" },
+    { url = "https://files.pythonhosted.org/packages/55/ef/b7c5ea0be82518906c978e365e56a77f8de7678c8bb6651ccfbdc178c29f/ruff-0.14.14-py3-none-musllinux_1_2_i686.whl", hash = "sha256:cc8b22da8d9d6fdd844a68ae937e2a0adf9b16514e9a97cc60355e2d4b219fc3", size = 10733525, upload-time = "2026-01-22T22:30:06.499Z" },
+    { url = "https://files.pythonhosted.org/packages/6a/5b/aaf1dfbcc53a2811f6cc0a1759de24e4b03e02ba8762daabd9b6bd8c59e3/ruff-0.14.14-py3-none-musllinux_1_2_x86_64.whl", hash = "sha256:16bc890fb4cc9781bb05beb5ab4cd51be9e7cb376bf1dd3580512b24eb3fda2b", size = 11315626, upload-time = "2026-01-22T22:30:36.848Z" },
+    { url = "https://files.pythonhosted.org/packages/2c/aa/9f89c719c467dfaf8ad799b9bae0df494513fb21d31a6059cb5870e57e74/ruff-0.14.14-py3-none-win32.whl", hash = "sha256:b530c191970b143375b6a68e6f743800b2b786bbcf03a7965b06c4bf04568167", size = 10502442, upload-time = "2026-01-22T22:30:38.93Z" },
+    { url = "https://files.pythonhosted.org/packages/87/44/90fa543014c45560cae1fffc63ea059fb3575ee6e1cb654562197e5d16fb/ruff-0.14.14-py3-none-win_amd64.whl", hash = "sha256:3dde1435e6b6fe5b66506c1dff67a421d0b7f6488d466f651c07f4cab3bf20fd", size = 11630486, upload-time = "2026-01-22T22:30:10.852Z" },
+    { url = "https://files.pythonhosted.org/packages/9e/6a/40fee331a52339926a92e17ae748827270b288a35ef4a15c9c8f2ec54715/ruff-0.14.14-py3-none-win_arm64.whl", hash = "sha256:56e6981a98b13a32236a72a8da421d7839221fa308b223b9283312312e5ac76c", size = 10920448, upload-time = "2026-01-22T22:30:15.417Z" },
+]
+
 [[package]]
 name = "sniffio"
 version = "1.3.1"
@@ -395,6 +421,24 @@ dev = [
     { name = "typing-extensions" },
     { name = "xonsh" },
 ]
+devx = [
+    { name = "greenback" },
+    { name = "stackscope" },
+    { name = "typing-extensions" },
+]
+lint = [
+    { name = "ruff" },
+]
+repl = [
+    { name = "prompt-toolkit" },
+    { name = "psutil" },
+    { name = "pyperclip" },
+    { name = "xonsh" },
+]
+testing = [
+    { name = "pexpect" },
+    { name = "pytest" },
+]
 
 [package.metadata]
 requires-dist = [
@@ -420,6 +464,22 @@ dev = [
     { name = "typing-extensions", specifier = ">=4.14.1" },
     { name = "xonsh", specifier = ">=0.19.2" },
 ]
+devx = [
+    { name = "greenback", specifier = ">=1.2.1,<2" },
+    { name = "stackscope", specifier = ">=0.2.2,<0.3" },
+    { name = "typing-extensions", specifier = ">=4.14.1" },
+]
+lint = [{ name = "ruff", specifier = ">=0.9.6" }]
+repl = [
+    { name = "prompt-toolkit", specifier = ">=3.0.50" },
+    { name = "psutil", specifier = ">=7.0.0" },
+    { name = "pyperclip", specifier = ">=1.9.0" },
+    { name = "xonsh", specifier = ">=0.19.2" },
+]
+testing = [
+    { name = "pexpect", specifier = ">=4.9.0,<5" },
+    { name = "pytest", specifier = ">=8.3.5" },
+]
 
 [[package]]
 name = "tricycle"