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

package.json

@@ -17,6 +17,8 @@
     "test": "node --test test/*.test.mjs",
     "build:visual-acceptance": "node tools/build-visual-acceptance.mjs",
     "test:phase4": "node --test test/phase4-*.test.mjs",
-    "test:phase5": "node test/phase5-cadence.test.mjs"
+    "test:phase5": "node test/phase5-cadence.test.mjs",
+    "test:phase6": "node test/phase6-scenarios.test.mjs",
+    "build:scenario-acceptance": "node tools/build-scenario-acceptance.mjs"
   }
 }