Add reproducible Nix dev environment

.gitignore

@@ -4,3 +4,4 @@
 .vscode
 lib/EpdFont/fontsrc
 *.generated.h
+result

flake.lock

@@ -0,0 +1,61 @@
+{
+  "nodes": {
+    "flake-parts": {
+      "inputs": {
+        "nixpkgs-lib": "nixpkgs-lib"
+      },
+      "locked": {
+        "lastModified": 1765835352,
+        "narHash": "sha256-XswHlK/Qtjasvhd1nOa1e8MgZ8GS//jBoTqWtrS1Giw=",
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "rev": "a34fae9c08a15ad73f295041fec82323541400a9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "hercules-ci",
+        "repo": "flake-parts",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1767379071,
+        "narHash": "sha256-EgE0pxsrW9jp9YFMkHL9JMXxcqi/OoumPJYwf+Okucw=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "fb7944c166a3b630f177938e478f0378e64ce108",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "nixpkgs-lib": {
+      "locked": {
+        "lastModified": 1765674936,
+        "narHash": "sha256-k00uTP4JNfmejrCLJOwdObYC9jHRrr/5M/a/8L2EIdo=",
+        "owner": "nix-community",
+        "repo": "nixpkgs.lib",
+        "rev": "2075416fcb47225d9b68ac469a5c4801a9c4dd85",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-community",
+        "repo": "nixpkgs.lib",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-parts": "flake-parts",
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

@@ -0,0 +1,120 @@
+{
+  description = "CrossPoint Reader - Firmware for Xteink X4 e-paper display";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-parts.url = "github:hercules-ci/flake-parts";
+  };
+
+  outputs = inputs@{ flake-parts, ... }:
+    flake-parts.lib.mkFlake { inherit inputs; } {
+      systems = [ "x86_64-linux" "aarch64-linux" "x86_64-darwin" "aarch64-darwin" ];
+
+      perSystem = { config, self', pkgs, lib, system, ... }:
+        let
+          # Version from platformio.ini
+          version = "0.12.0";
+
+          # Create a platformio wrapper with all required dependencies
+          platformio = pkgs.platformio-core.overrideAttrs (old: {
+            propagatedBuildInputs = (old.propagatedBuildInputs or [ ]) ++ [
+              pkgs.python3Packages.packaging
+            ];
+          });
+
+          # Python with required packages for build scripts
+          pythonEnv = pkgs.python3.withPackages (ps: [ ps.pip ]);
+
+          # FHS environment to run PlatformIO with dynamically linked toolchains
+          fhsEnv = pkgs.buildFHSEnv {
+            name = "pio-env";
+            targetPkgs = pkgs: [
+              platformio
+              pythonEnv
+              pkgs.git
+              pkgs.zlib
+              pkgs.stdenv.cc.cc.lib
+            ];
+            runScript = "bash";
+          };
+
+          # Build script that runs inside FHS environment
+          buildScript = pkgs.writeShellScriptBin "build-firmware" ''
+            set -e
+            echo "Building CrossPoint Reader firmware..."
+            echo ""
+
+            # Ensure we're in the project directory
+            if [ ! -f platformio.ini ]; then
+              echo "Error: platformio.ini not found. Run this from the project root."
+              exit 1
+            fi
+
+            # Run the build inside FHS environment
+            ${fhsEnv}/bin/pio-env -c "pio run -e gh_release"
+
+            echo ""
+            echo "Build complete! Firmware at: .pio/build/gh_release/firmware.bin"
+          '';
+
+          # Wrapper for pio that runs inside FHS environment
+          pioWrapper = pkgs.writeShellScriptBin "pio" ''
+            exec ${fhsEnv}/bin/pio-env -c "pio $*"
+          '';
+        in
+        {
+          packages = {
+            default = buildScript;
+            build-script = buildScript;
+          };
+
+          devShells.default = pkgs.mkShell {
+            packages = [
+              # Build tools (wrapped for FHS compatibility)
+              fhsEnv
+              pioWrapper
+              buildScript
+              pythonEnv
+
+              # C/C++ language server
+              pkgs.clang-tools
+
+              # Python language server
+              pkgs.python3Packages.python-lsp-server
+
+              # Nix language server
+              pkgs.nixd
+
+              # Git for submodules
+              pkgs.git
+
+              # Useful utilities
+              pkgs.esptool
+            ];
+
+            shellHook = ''
+              echo "CrossPoint Reader development environment"
+              echo ""
+              echo "Build commands:"
+              echo "  pio run               - Build firmware (default env, via FHS wrapper)"
+              echo "  pio run -e gh_release - Build release firmware"
+              echo "  pio run --target upload - Flash to device"
+              echo "  build-firmware        - Build release firmware directly"
+              echo ""
+              echo "Enter FHS shell for direct PlatformIO access:"
+              echo "  pio-env               - Enter FHS shell with PlatformIO"
+              echo ""
+              echo "Language servers available:"
+              echo "  clangd   - C/C++ LSP"
+              echo "  pylsp    - Python LSP"
+              echo "  nixd     - Nix LSP"
+            '';
+          };
+
+          apps.build = {
+            type = "app";
+            program = "${buildScript}/bin/build-firmware";
+          };
+        };
+    };
+}