seat: implement per-device keymaps

toml-spec/spec/spec.generated.json

@@ -790,6 +790,10 @@
               "description": ""
             }
           }
+        },
+        "keymap": {
+          "description": "The keymap to use for this device.\n\nThis overrides the global keymap. The keymap becomes active when a key is pressed.\n\n- Example:\n\n  ```toml\n  [[inputs]]\n  match.name = \"ZSA Technology Labs Inc ErgoDox EZ\"\n  keymap.name = \"external\"\n  ```\n",
+          "$ref": "#/$defs/Keymap"
         }
       },
       "required": [