Ensure DDS_SCHEDULER path compiles in Linux CI

[tameware]

Summary

• Add a Linux CI step that builds //library/src:dds with --define=scheduler=true so the gated DDS_SCHEDULER timing path stays compile-clean.
• Document the step in docs/BUILD_SYSTEM.md.

Test plan

• Confirm Linux CI build_and_test runs the new scheduler define build step
• Confirm bazelisk build --define=scheduler=true //library/src:dds succeeds locally

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