build(visual): bundle the visual contract and validation modules into the standalone artifact

The standalone build's source list is explicit, so the two modules the previous
commit added were called by validator.js inside XZBT.html without being defined
there. Both are now bundled ahead of validator.js and resolution.js, which are
their only consumers.

visual-validation.js used a local isPlainObject that would have collided with
validator.js's own once the bundler strips imports and concatenates into one
scope; it uses the shared isRecord of types.js instead.

The rebuilt artifact parses and carries validateVisualSubsystem and
matchVisualTarget. npm test remains at 131 passing.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_0162Jb1J36judZNT8fHabGVt

tools/build-xzbt.mjs

@@ -16,6 +16,8 @@ const sourceFiles = Object.freeze([
   'src/runtime/audio-automation.js',
   'src/runtime/audio-graph.js',
   'src/runtime/audio-controls.js',
+  'src/runtime/visual-contract.js',
+  'src/runtime/visual-validation.js',
   'src/runtime/validator.js',
   'src/runtime/persistence.js',
   'src/runtime/library.js',