Problem

Consumer breakdown can get stuck in "Calculating..." state forever when the Web Worker hangs and never resolves/rejects its promise.

Root cause: No timeout on tokenWorker.calculate() means if the worker hangs:

• Promise never settles
• Code never reaches catch/finally blocks
• pendingCalcs never cleaned up
• UI stuck showing "Calculating..." indefinitely

This was observed during development when errors occurred.

Solution

Add a 10-second timeout using Promise.race:

const timeoutPromise = new Promise<never>((_, reject) =>
  setTimeout(() => reject(new Error("Calculation timeout")), 10_000)
);

const fullStats = await Promise.race([
  this.tokenWorker.calculate(messages, model),
  timeoutPromise,
]);

Why 10 seconds?

• Generous enough for large message histories
• Responsive - anything longer feels like a bug to users
• Treated as error → caches empty result → prevents retry spam

Unchanged: Original Cancellation Logic

The existing cancellation handling is preserved and correct:

catch (error) {
  if (error.message === "Cancelled by newer request") {
    return; // Don't cache, let lazy trigger retry
  }
  // Real errors: cache empty to prevent infinite retries
  this.cache.set(workspaceId, { ..., isCalculating: false });
}

Why this matters:

• Cancellations (transient): Don't cache → Lazy trigger retries naturally
• Real errors (permanent): Cache empty → Prevent retry spam
• Removing this distinction breaks the system

Net Changes

• Added: 6 lines (timeout constant + Promise.race wrapper)
• Removed: 0 lines
• Net: +6 lines, 0 complexity increase

Testing

• ✅ Fresh load with large history (timeout doesn't trigger prematurely)
• ✅ Rapid workspace switching (cancellations still work correctly)
• ✅ Simulated worker hang (timeout fires, UI exits calculating state)

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