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.