fix(core): emit mediaVolumeEnvelope in tsc lib output (#1159)

core's tsconfig excludes src/runtime (it's the browser IIFE bundle
source), so tsc never emitted dist/runtime/mediaVolumeEnvelope.{js,d.ts}.
The publishConfig export ./media-volume-envelope pointed at that missing
dist file — invisible in-monorepo (dev export resolves via src) but
breaks external consumers that resolve the published export.

Fix: add the file to tsconfig's `files` array, which overrides `exclude`
per the TypeScript spec. The IIFE bundler (esbuild) is unaffected — it
resolves from the entry point, not tsconfig. No package.json or import
changes needed.

packages/core/tsconfig.json

@@ -13,6 +13,7 @@
     "outDir": "./dist",
     "rootDir": "./src"
   },
+  "files": ["src/runtime/mediaVolumeEnvelope.ts"],
   "include": ["src/**/*"],
   "exclude": ["node_modules", "dist", "src/tests", "src/runtime", "**/*.test.ts"]
 }