bench: add A/B branch comparison workflow to BENCHMARKING.md

[timneutkens]

What

Adds a new "A/B branch comparison" section to bench/BENCHMARKING.md with practical guidance for comparing benchmark results across two branches.

Why

From running benchmarks comparing canary vs PR #92678, we identified several patterns that lead to noisy or misleading results:

• Running the full route suite when only one route shows signal wastes time (~3 min per run)
• The default 120 serial requests is too noisy for sub-2ms routes
• Single runs can swing 10-15% on light routes
• Percentage deltas from a single pair of runs can be misleading without absolute numbers

What's in the new section

• Start with a focused route, not the full suite
• Increase request counts for fast routes (500 serial / 5000 load)
• Run at least 3 times per side to average out noise
• Compare absolute req/s, not just deltas
• Watch for system state drift between runs
• Full example workflow with checkout/build/run loop

[timneutkens]

• bench: add A/B branch comparison workflow to BENCHMARKING.md #92721 👈 (View in Graphite)
• canary

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