feat: implement Phase 1 TCP client

flake.nix

@@ -0,0 +1,37 @@
+{
+  description = "fumi - Smol Mail client (Rust)";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    flake-utils.lib.eachDefaultSystem (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+        fumi = { rns ? false }: pkgs.rustPlatform.buildRustPackage {
+          pname = "fumi";
+          version = "0.1.0";
+          src = ./.;
+          cargoLock.lockFile = ./Cargo.lock;
+          buildFeatures = pkgs.lib.optionals rns [ "rns" ];
+          # The rns feature vendors microReticulum behind the shim and needs
+          # MICRORETICULUM_SOURCE_DIR; it is added with the RNS carrier.
+          doCheck = !rns;
+        };
+      in
+      {
+        packages.default = fumi { };
+        packages.rns = fumi { rns = true; };
+
+        apps.default = flake-utils.lib.mkApp {
+          drv = fumi { };
+          name = "fumi";
+        };
+
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [ cargo rustc rustfmt clippy sqlite ];
+        };
+      });
+}