feat(scenario): implement the phase 6 scenario director

Add Format Specification section 21 (Scenario Model 0.1) and the runtime
that realizes it: the scenario director, seven trigger classes, timelines
with repeats and branches, admission and concurrency control, nested
ownership with bounded cleanup, and the production GC5 resource counters.

Exhibit E provides a reproducible forty-minute long-scenario reference and
scenario-challenge.xzbt covers the PRD 131 cases. A standalone
acceptance/soak page is built by tools/build-scenario-acceptance.mjs.

252 automated checks pass. The two-hour real-duration development soak
required by the GC6 schedule remains pending.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01M7dgfQ12mpM4JjSMv3inLA

exhibits/exhibit-b.xzbt

@@ -4,7 +4,7 @@
     "id": "exhibit-b",
     "name": "Exhibit B — Deep Abstract Field",
     "version": "0.1.0",
-    "description": "First visual composition; full reference exhibit acceptance remains Phase 9."
+    "description": "Procedural audiovisual reference with cadence and a temporary scenario override; final acceptance remains Phase 9."
   },
   "runtime": {
     "seed": 42
@@ -174,5 +174,166 @@
         "intensity": 0.5
       }
     ]
+  },
+  "parameters": {
+    "activity": {
+      "type": "number",
+      "default": 0.4,
+      "min": 0,
+      "max": 1,
+      "label": "Activity"
+    }
+  },
+  "bindings": [
+    {
+      "source": "parameters.activity",
+      "target": "visuals.camera.zoom",
+      "scale": 0.25,
+      "offset": 1
+    }
+  ],
+  "cadence": {
+    "intensity": {
+      "ref": "parameters.activity"
+    }
+  },
+  "audio": {
+    "buses": {
+      "ambient": {
+        "gain": 0.5
+      },
+      "effects": {
+        "gain": 0.5
+      }
+    }
+  },
+  "sounds": {
+    "bed": {
+      "name": "Distant hum",
+      "bus": "ambient",
+      "usage": [
+        "automatic"
+      ],
+      "cadence": {
+        "class": "ambient"
+      },
+      "recipe": {
+        "mode": "continuous",
+        "release": "1s",
+        "nodes": {
+          "tone": {
+            "type": "oscillator",
+            "frequency": 55
+          },
+          "trim": {
+            "type": "gain",
+            "gain": 0.04
+          }
+        },
+        "routes": [
+          {
+            "from": "tone",
+            "to": "trim"
+          },
+          {
+            "from": "trim",
+            "to": "output"
+          }
+        ]
+      }
+    },
+    "drone": {
+      "name": "Passing resonance",
+      "bus": "ambient",
+      "usage": [
+        "scenario"
+      ],
+      "recipe": {
+        "mode": "continuous",
+        "release": "2s",
+        "nodes": {
+          "tone": {
+            "type": "oscillator",
+            "frequency": 165
+          },
+          "trim": {
+            "type": "gain",
+            "gain": 0.035
+          }
+        },
+        "routes": [
+          {
+            "from": "tone",
+            "to": "trim"
+          },
+          {
+            "from": "trim",
+            "to": "output"
+          }
+        ]
+      }
+    },
+    "beat": {
+      "name": "Soft impulse",
+      "bus": "effects",
+      "usage": [
+        "automatic",
+        "scenario",
+        "manual"
+      ],
+      "cadence": {
+        "class": "routine",
+        "overlap": false,
+        "cooldown": "3s"
+      },
+      "recipe": {
+        "mode": "oneshot",
+        "release": "50ms",
+        "nodes": {
+          "hit": {
+            "type": "impulse",
+            "duration": "8ms",
+            "amplitude": 0.06
+          }
+        },
+        "routes": [
+          {
+            "from": "hit",
+            "to": "output"
+          }
+        ]
+      }
+    }
+  },
+  "scenarios": {
+    "disturbance": {
+      "name": "Temporary disturbance",
+      "trigger": {
+        "type": "random-interval",
+        "min": "30s",
+        "max": "90s"
+      },
+      "duration": "12s",
+      "cooldown": "20s",
+      "priority": 70,
+      "onStart": [
+        {
+          "type": "override",
+          "target": "parameters.activity",
+          "value": 0.95,
+          "scope": "scenario",
+          "transition": {
+            "in": "1s",
+            "out": "2s",
+            "easing": "ease-in-out"
+          }
+        },
+        {
+          "type": "sound",
+          "sound": "drone"
+        }
+      ],
+      "timeline": []
+    }
   }
 }