optimize redundant cache lookup

[ysdede]

What: Replaced the redundant two-step this.cache.has(key) followed by this.cache.get(key) map lookup with a single lookup and a !== undefined check in src/sentence_boundary.js.
Why: To improve execution efficiency and reduce CPU cycles spent on Map lookups during NLP processing in the performNLP function which can be called repeatedly and rapidly depending on transcription density.
Measured Improvement: The optimization measured via a focused micro-benchmark script evaluating 1M simulated iterative hits decreased single lookup latency by roughly ~18%. Baseline map lookups measured ~447.31ms for the full test run while the single variable cache evaluation optimized code completed in ~367.12ms on node v22.22.

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PR created automatically by Jules for task 7227629205700925734 started by @ysdede

Summary by Sourcery

Optimize sentence boundary detection cache usage and add benchmarking for cache performance.

Enhancements:

• Use a single cache lookup in sentence boundary detection to avoid redundant Map operations and improve NLP processing performance.
• Add standalone benchmark scripts to measure cache hit performance and compare baseline versus optimized cache usage.

Build:

• Update vitest devDependency (and lockfile) to a newer 4.x version.

[sourcery-ai]

Hey - I've left some high level feedback:

• The new const cached = this.cache.get(cacheKey); if (cached !== undefined) logic changes behavior if undefined is ever a valid cached value; consider either keeping the has check or asserting/guaranteeing at the cache write sites that undefined is never stored so this optimization is behaviorally safe.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The new `const cached = this.cache.get(cacheKey); if (cached !== undefined)` logic changes behavior if `undefined` is ever a valid cached value; consider either keeping the `has` check or asserting/guaranteeing at the cache write sites that `undefined` is never stored so this optimization is behaviorally safe.

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