Adds a 2-worker pool in use-syntax-highlight.ts so shiki tokenisation
runs off the main thread. Callers continue to receive null while the
worker is in flight and a LineTokenMap once it resolves — no caller
changes needed.
Fallback: if Worker construction is blocked (e.g. CSP), the hook falls
back to the existing createHighlighter singleton but processes 200 lines
at a time, yielding between chunks via scheduler.yield()/setTimeout(0)
to avoid long tasks.
Also adds worker.format:'es' to vite.config.ts (required when the app
uses code-splitting) and covers all paths with Vitest tests.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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