bench(text-buffer-view): add measureForDimensions benchmark

[simonklee]

Add a benchmark for measureForDimensions().

This function creates a fresh arena and performs a full rope walk on every call.

The benchmark has two scenarios:

• static: repeated calls on unchanged content (measures cache-miss penalty)
• streaming: calls interleaved with token appends (simulates LLM output)

This adds a baseline for measuring future fixes like result caching, arena reuse.

Refs: anomalyco/opencode#6172

[kommander]

Thanks, yeah that is awful.