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

tools/build-xzbt.mjs

@@ -27,6 +27,7 @@ const sourceFiles = Object.freeze([
   'src/runtime/visual-systems.js',
   'src/runtime/visual-validation.js',
   'src/runtime/cadence-validation.js',
+  'src/runtime/scenario-validation.js',
   'src/runtime/validator.js',
   'src/runtime/persistence.js',
   'src/runtime/library.js',
@@ -40,6 +41,7 @@ const sourceFiles = Object.freeze([
   'src/runtime/visual-subsystem.js',
   'src/runtime/actions.js',
   'src/runtime/cadence.js',
+  'src/runtime/scenario.js',
   'src/runtime/performance.js',
   'src/runtime/activation.js',
   'src/runtime/audio-engine.js',