Initial System76 Launch 3 keyboard firmware project

Nix flake with devShell providing qmk CLI for firmware development.
Custom keymap (copy of default) symlinked into QMK tree on shell entry.

Setup: nix develop, then qmk setup system76/qmk_firmware
Build: qmk compile -kb system76/launch_3 -km custom

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1773282481,
+        "narHash": "sha256-b/GV2ysM8mKHhinse2wz+uP37epUrSE+sAKXy/xvBY4=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "fe416aaedd397cacb33a610b33d60ff2b431b127",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}