fix: harden worklet caching and build guard

Un-hashed libopenmpt.worklet.js was inheriting the immutable
1-year cache from /assets/, risking stale audio on chiptune3
bumps. Add a dedicated short-cache location for it.

Also fail closeBundle loudly if the worklet source vanishes,
instead of silently shipping a build with hanging audio.

vite.config.ts

@@ -30,7 +30,11 @@ function chiptuneWorkletFix(): Plugin {
     },
     closeBundle() {
       const src = resolve('node_modules/chiptune3/libopenmpt.worklet.js')
-      if (!existsSync(src)) return
+      if (!existsSync(src)) {
+        throw new Error(
+          'chiptuneWorkletFix: libopenmpt.worklet.js not found — chiptune3 layout changed?',
+        )
+      }
       const dest = resolve(outDir, assetsDir, 'libopenmpt.worklet.js')
       copyFileSync(src, dest)
     },