docs: add benchmark segmentation guidance to prevent agent timeouts

[LeeCampbell]

Summary

• Add timeout warning to testing standards: full benchmark suites exceed the 30-minute agent iteration timeout
• Document three segmentation strategies (by class, by runtime, or combined) with concrete command examples
• Add timeout keyword to spec README index for discoverability

Test plan

• Verify the documented --filter and --runtimes flags work with dotnet run commands
• Confirm a single-class, single-runtime benchmark run completes well within 30 minutes

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