Add dashboard subscriber queue and performance controls

[dallison]

Summary

• add subscriber queue-size controls that recreate channels with matching queue arenas
• add throughput and latency operating modes with mode-specific notification and read strategies
• add 2 MHz and 5 MHz rate presets
• harden curses input handling on macOS and document the expanded dashboard controls

Testing

• bazelisk test //dashboard:dashboard_test //dashboard:engine_test --config=macos_arm64 -c opt --test_output=errors
• bazelisk test //client:client_test --config=macos_arm64 -c opt --test_output=errors
• exercised throughput/latency switching in the optimized curses UI