add nix dev shell and setup notes

DEV_SETUP.md

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+# Development setup
+
+## Enter shell
+
+```bash
+nix develop
+```
+
+This shell includes:
+
+- Rust: `cargo`, `rustc`, `rustfmt`, `clippy`, `rust-analyzer`
+- Build tools: `pkg-config`, `gcc`, `clang`, `make`, `git`
+- Input debug: `evtest`, `libinput`, `wev`
+- CLI tools: `jq`, `rg`, `fd`, `jj`
+
+## Recommended NixOS runtime config (uinput)
+
+For `gotta-scroll-fast` (`gsfd`), you typically need `/dev/uinput`.
+
+```nix
+{
+  boot.kernelModules = [ "uinput" ];
+
+  # Optional: ensure module is available in initrd/system profile
+  boot.extraModulePackages = [ ];
+
+  services.udev.extraRules = ''
+    KERNEL=="uinput", MODE="0660", GROUP="input"
+  '';
+
+  users.users.<your-user>.extraGroups = [ "input" ];
+}
+```
+
+Then rebuild and re-login.
+
+## Useful checks
+
+```bash
+ls -l /dev/uinput
+id
+sudo evtest
+libinput debug-events
+wev
+```
+
+## First run (project)
+
+```bash
+cargo run -p gsf-daemon -- --list-devices
+cargo run -p gsf-cli -- tui
+```

flake.lock

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+{
+  "nodes": {
+    "flake-utils": {
+      "inputs": {
+        "systems": "systems"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1774106199,
+        "narHash": "sha256-US5Tda2sKmjrg2lNHQL3jRQ6p96cgfWh3J1QBliQ8Ws=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "6c9a78c09ff4d6c21d0319114873508a6ec01655",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": "nixpkgs"
+      }
+    },
+    "systems": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}

flake.nix

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+{
+  description = "gotta-scroll-fast (gsf) - host-side scroll acceleration toolkit";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    flake-utils.url = "github:numtide/flake-utils";
+  };
+
+  outputs = { self, nixpkgs, flake-utils }:
+    let
+      eachSystem = flake-utils.lib.eachDefaultSystem;
+    in
+    eachSystem (system:
+      let
+        pkgs = import nixpkgs {
+          inherit system;
+          config.allowUnfree = false;
+        };
+      in {
+        devShells.default = pkgs.mkShell {
+          packages = with pkgs; [
+            rustc
+            cargo
+            rustfmt
+            clippy
+            rust-analyzer
+            rustPlatform.rustLibSrc
+
+            pkg-config
+            gcc
+            clang
+            gnumake
+            git
+
+            evtest
+            libinput
+            wev
+
+            jq
+            ripgrep
+            fd
+            jj
+          ];
+
+          shellHook = ''
+            export RUST_SRC_PATH="${pkgs.rustPlatform.rustLibSrc}"
+
+            echo
+            echo "✅ gsf dev shell ready"
+            echo "   Rust: $(rustc --version)"
+            echo "   Cargo: $(cargo --version)"
+            echo "   JJ: $(jj --version | head -n1)"
+            echo
+            if [ -e /dev/uinput ]; then
+              echo "uinput: present at /dev/uinput"
+              ls -l /dev/uinput
+            else
+              echo "uinput: /dev/uinput not found"
+              echo "        enable with: boot.kernelModules = [ \"uinput\" ];"
+            fi
+            echo
+          '';
+        };
+      });
+}