feat(phase0): complete GC1 feasibility and GC2 shared format contracts

- Record user evidence for directory fallback to complete GC1 (10/10 checks passed)

- Expand Format Specification 0.1 to Revision 0.2 with normative shared contracts

- Author JSON Schema Draft-07 at schema/xzbt-0.1.schema.json

- Implement zero-dependency semantic validator at tools/validate-exhibit.mjs

- Create 12-case conformance fixture suite and automated test runner (12/12 passing)

- Update implementation status, verification gates, and gap closure decisions

- Add devlog entry covering stall recovery and Phase 0 current state

test/fixtures/gc2/invalid-cyclic-bindings.xzbt

@@ -0,0 +1,29 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "cyclic-bindings-study",
+    "name": "Invalid Cyclic Bindings Study"
+  },
+  "parameters": {
+    "activity": {
+      "type": "number",
+      "default": 0.5
+    }
+  },
+  "state": {
+    "energy": {
+      "type": "number",
+      "initial": 0.5
+    }
+  },
+  "bindings": [
+    {
+      "from": "parameters.activity",
+      "to": "state.energy"
+    },
+    {
+      "from": "state.energy",
+      "to": "parameters.activity"
+    }
+  ]
+}

test/fixtures/gc2/invalid-id-syntax.xzbt

@@ -0,0 +1,7 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "Valid-Id-Upper",
+    "name": "Invalid ID Syntax Study"
+  }
+}

test/fixtures/gc2/invalid-operator-arity.xzbt

@@ -0,0 +1,31 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "operator-arity-study",
+    "name": "Invalid Operator Arity Study"
+  },
+  "parameters": {
+    "activity": {
+      "type": "number",
+      "default": 0.5
+    }
+  },
+  "state": {
+    "energy": {
+      "type": "number",
+      "initial": 0.5
+    }
+  },
+  "bindings": [
+    {
+      "from": "parameters.activity",
+      "to": "state.energy",
+      "transform": {
+        "op": "add",
+        "args": [
+          1.0
+        ]
+      }
+    }
+  ]
+}

test/fixtures/gc2/invalid-out-of-bounds.xzbt

@@ -0,0 +1,15 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "out-of-bounds-study",
+    "name": "Invalid Out of Bounds Study"
+  },
+  "parameters": {
+    "activity": {
+      "type": "number",
+      "default": 1.5,
+      "min": 0.0,
+      "max": 1.0
+    }
+  }
+}

test/fixtures/gc2/invalid-reference-missing.xzbt

@@ -0,0 +1,19 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "missing-ref-study",
+    "name": "Invalid Reference Missing Study"
+  },
+  "parameters": {
+    "activity": {
+      "type": "number",
+      "default": 0.5
+    }
+  },
+  "bindings": [
+    {
+      "from": "parameters.non_existent_param",
+      "to": "parameters.activity"
+    }
+  ]
+}

test/fixtures/gc2/invalid-type-mismatch.xzbt

@@ -0,0 +1,13 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "type-mismatch-study",
+    "name": "Invalid Type Mismatch Study"
+  },
+  "parameters": {
+    "activity": {
+      "type": "number",
+      "default": "0.5"
+    }
+  }
+}

test/fixtures/gc2/invalid-unknown-field.xzbt

@@ -0,0 +1,14 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "unknown-field-study",
+    "name": "Invalid Unknown Field Study"
+  },
+  "parameters": {
+    "speed": {
+      "type": "number",
+      "default": 1.0,
+      "unrecognized_property": true
+    }
+  }
+}

test/fixtures/gc2/invalid-unsupported-version.xzbt

@@ -0,0 +1,7 @@
+{
+  "xzbt": "0.99-unsupported",
+  "meta": {
+    "id": "unsupported-version-study",
+    "name": "Invalid Version Study"
+  }
+}

test/fixtures/gc2/valid-full-feature.xzbt

@@ -0,0 +1,63 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "full-study",
+    "name": "Full Feature Conformance Study",
+    "version": "1.0.0",
+    "author": "Antigravity",
+    "description": "Valid exhibit exercising parameters, state, expressions, and bindings.",
+    "tags": ["test", "conformance", "gc2"]
+  },
+  "runtime": {
+    "seed": 42
+  },
+  "parameters": {
+    "activity": {
+      "type": "number",
+      "default": 0.5,
+      "min": 0.0,
+      "max": 1.0,
+      "step": 0.01,
+      "label": "Activity Level"
+    },
+    "high-contrast": {
+      "type": "boolean",
+      "default": false
+    },
+    "wave-shape": {
+      "type": "enum",
+      "values": ["sine", "triangle", "square"],
+      "default": "sine"
+    },
+    "accent-color": {
+      "type": "color",
+      "default": "#e8ad57"
+    }
+  },
+  "state": {
+    "energy": {
+      "type": "number",
+      "initial": 0.2,
+      "min": 0.0,
+      "max": 1.0
+    },
+    "alert-active": {
+      "type": "boolean",
+      "initial": false
+    }
+  },
+  "bindings": [
+    {
+      "from": "parameters.activity",
+      "to": "state.energy",
+      "transform": {
+        "op": "multiply",
+        "args": [
+          { "ref": "parameters.activity" },
+          1.5
+        ]
+      },
+      "smoothing": "250ms"
+    }
+  ]
+}

test/fixtures/gc2/valid-minimal.xzbt

@@ -0,0 +1,7 @@
+{
+  "xzbt": "0.1",
+  "meta": {
+    "id": "minimal-study",
+    "name": "Minimal Valid Study"
+  }
+}