Clarify GUI notification thread contracts

[satoshi-assistant]

Summary

• Update wrac-plugin-template to include the merged GUI thread-safety contract documentation
• Remove the extra RunLoopSender requirement from example GUI notifier subscriptions
• Document that resize commands enter the host GUI resize extension from the GUI event path
• Fix a clippy warning in the sine-synth audio renderer

Verification

• cargo fmt --all -- --check
• cargo check -p wrac_gain_basic_plugin -p wrac_sine_synth_plugin
• cargo clippy -p wrac_gain_basic_plugin -p wrac_sine_synth_plugin --all-targets --all-features -- -D warnings
• cargo test -p wrac_gain_basic_plugin -p wrac_sine_synth_plugin --all-features