feat(js): m01 util and m02 model with tests, fixtures, purity audit

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>

fixtures/m02-delta.json

@@ -0,0 +1,45 @@
+{
+  "function": "delta",
+  "cases": [
+    {
+      "name": "up",
+      "args": [
+        "Up"
+      ],
+      "expected": {
+        "x": 0,
+        "y": -1
+      }
+    },
+    {
+      "name": "down",
+      "args": [
+        "Down"
+      ],
+      "expected": {
+        "x": 0,
+        "y": 1
+      }
+    },
+    {
+      "name": "left",
+      "args": [
+        "Left"
+      ],
+      "expected": {
+        "x": -1,
+        "y": 0
+      }
+    },
+    {
+      "name": "right",
+      "args": [
+        "Right"
+      ],
+      "expected": {
+        "x": 1,
+        "y": 0
+      }
+    }
+  ]
+}

fixtures/m02-initial-state.json

@@ -0,0 +1,67 @@
+{
+  "function": "initialState",
+  "cases": [
+    {
+      "name": "seed-7",
+      "args": [
+        7
+      ],
+      "expected": {
+        "width": 20,
+        "height": 20,
+        "snake": [
+          {
+            "x": 10,
+            "y": 10
+          },
+          {
+            "x": 9,
+            "y": 10
+          },
+          {
+            "x": 8,
+            "y": 10
+          }
+        ],
+        "dir": "Right",
+        "nextDir": "Right",
+        "apple": null,
+        "seed": 7,
+        "score": 0,
+        "status": "Playing",
+        "frame": 0
+      }
+    },
+    {
+      "name": "seed-is-stored-as-given",
+      "args": [
+        4294967295
+      ],
+      "expected": {
+        "width": 20,
+        "height": 20,
+        "snake": [
+          {
+            "x": 10,
+            "y": 10
+          },
+          {
+            "x": 9,
+            "y": 10
+          },
+          {
+            "x": 8,
+            "y": 10
+          }
+        ],
+        "dir": "Right",
+        "nextDir": "Right",
+        "apple": null,
+        "seed": 4294967295,
+        "score": 0,
+        "status": "Playing",
+        "frame": 0
+      }
+    }
+  ]
+}

fixtures/m02-opposite.json

@@ -0,0 +1,33 @@
+{
+  "function": "opposite",
+  "cases": [
+    {
+      "name": "up",
+      "args": [
+        "Up"
+      ],
+      "expected": "Down"
+    },
+    {
+      "name": "down",
+      "args": [
+        "Down"
+      ],
+      "expected": "Up"
+    },
+    {
+      "name": "left",
+      "args": [
+        "Left"
+      ],
+      "expected": "Right"
+    },
+    {
+      "name": "right",
+      "args": [
+        "Right"
+      ],
+      "expected": "Left"
+    }
+  ]
+}

fixtures/m02-point-equals.json

@@ -0,0 +1,61 @@
+{
+  "function": "pointEquals",
+  "cases": [
+    {
+      "name": "same-point",
+      "args": [
+        {
+          "x": 3,
+          "y": 4
+        },
+        {
+          "x": 3,
+          "y": 4
+        }
+      ],
+      "expected": true
+    },
+    {
+      "name": "different-x",
+      "args": [
+        {
+          "x": 3,
+          "y": 4
+        },
+        {
+          "x": 2,
+          "y": 4
+        }
+      ],
+      "expected": false
+    },
+    {
+      "name": "different-y",
+      "args": [
+        {
+          "x": 3,
+          "y": 4
+        },
+        {
+          "x": 3,
+          "y": 5
+        }
+      ],
+      "expected": false
+    },
+    {
+      "name": "swapped-coordinates",
+      "args": [
+        {
+          "x": 3,
+          "y": 4
+        },
+        {
+          "x": 4,
+          "y": 3
+        }
+      ],
+      "expected": false
+    }
+  ]
+}