Add Nix flake dev shell and NixOS SPI setup docs

RaspberryPi_JetsonNano/c/NIX_CONFIG.md

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+# NixOS Configuration for Waveshare e-Paper (RaspberryPi_JetsonNano/c)
+
+This project now includes a Nix flake dev shell (`flake.nix` + `flake.lock`) that builds the C examples on NixOS.
+
+## 1. Use the dev shell without copying the whole git repo
+
+From `RaspberryPi_JetsonNano/c`, use:
+
+```bash
+nix develop path:.
+```
+
+Do not use plain `nix develop .` in this mono-repo, because Nix may treat it as a git flake rooted at `/home/.../e-Paper` and spend time copying the full repo to the store.
+
+## 2. Build command
+
+Inside the dev shell:
+
+```bash
+make -j4 EPD=epd5in65f
+```
+
+`flake.nix` exports `MAKEFLAGS=USELIB_RPI=USE_DEV_LIB` so default builds use the `libgpiod` backend (compatible with this environment).
+
+## 3. Persistent NixOS system config (SPI + non-root access)
+
+`boot.loader.raspberryPi` was removed from nixpkgs and no longer works.
+Use `raspberry-pi-nix` for modern Raspberry Pi boot/device-tree configuration.
+
+### Option A: Use the dedicated image repo (recommended)
+
+Use `https://github.com/anicolao/nix-epaper`, which already includes:
+- SPI firmware configuration
+- `spidev` kernel module
+- `spi`/`gpio` groups and udev rules
+
+### Option B: Add equivalent settings to your own system flake
+
+In your system `flake.nix`:
+
+```nix
+{
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    raspberry-pi-nix = {
+      url = "github:nix-community/raspberry-pi-nix";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+  };
+}
+```
+
+In your NixOS configuration module:
+
+```nix
+{
+  imports = [ raspberry-pi-nix.nixosModules.raspberry-pi ];
+
+  # Pi 4; use "bcm2712" for Pi 5.
+  raspberry-pi-nix.board = "bcm2711";
+
+  # Enable SPI in generated config.txt.
+  hardware.raspberry-pi.config.all.base-dt-params.spi = {
+    enable = true;
+    value = "on";
+  };
+
+  # Ensure userspace SPI char device and permissions.
+  boot.kernelModules = [ "spidev" ];
+  users.groups.spi = {};
+  users.groups.gpio = {};
+  users.users.anicolao.extraGroups = [ "spi" "gpio" ];
+  services.udev.extraRules = ''
+    KERNEL=="spidev*", GROUP="spi", MODE="0660"
+    KERNEL=="gpiochip*", GROUP="gpio", MODE="0660"
+  '';
+}
+```
+
+Apply and reboot:
+
+```bash
+sudo nixos-rebuild switch
+sudo reboot
+```
+
+After reboot, verify:
+
+```bash
+ls -l /dev/spidev* /dev/gpiochip*
+id
+```
+
+You should see `/dev/spidev0.0` and your user in `spi` + `gpio` groups.
+
+## 4. Why this is needed
+
+- `sudo ./epd` previously failed with `/dev/spidev0.0: No such file or directory`.
+- That indicates SPI bus nodes were not enabled by firmware/device-tree.
+- `modprobe spidev` alone is temporary and does not set persistent permissions/groups.

RaspberryPi_JetsonNano/c/flake.nix

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+{
+  description = "Waveshare e-Paper C dev shell for NixOS";
+
+  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;
+        };
+      in {
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            bun
+            gcc
+            gnumake
+            pkg-config
+            linuxHeaders
+            libgpiod_1
+            wiringpi
+          ];
+
+          shellHook = ''
+            export MAKEFLAGS="USELIB_RPI=USE_DEV_LIB"
+            echo "Waveshare e-Paper C dev shell"
+            echo "Build example: make clean && make -j4 EPD=epd2in13V4"
+          '';
+        };
+      });
+}