Add Noise Gate

[OpenSauce]

Closes #22

[Copilot]

Pull Request Overview

This PR implements a noise gate stage for the audio processing chain to eliminate unwanted noise when not playing. The noise gate features configurable threshold, ratio, attack, hold, and release parameters with smooth transitions to avoid abrupt audio cuts.

Key changes:

• Implements a complete NoiseGateStage with envelope following and smooth gating transitions
• Adds GUI configuration and controls for all noise gate parameters
• Integrates the noise gate into the existing stage system with proper message handling

Reviewed Changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/sim/stages/noise_gate.rs	Core noise gate implementation with envelope tracking and parameter controls
src/sim/stages/mod.rs	Exports the new noise_gate module
src/gui/messages.rs	Adds NoiseGateMessage enum for GUI communication
src/gui/config/noise_gate.rs	Configuration struct with default values and stage conversion
src/gui/config/mod.rs	Integrates noise gate config into the stage system
src/gui/components/stages/noise_gate.rs	GUI component with sliders for all parameters
src/gui/components/stages/mod.rs	Exports the noise gate GUI component
src/gui/components/stage_list.rs	Renders noise gate stages in the stage list
src/gui/components/control.rs	Adds noise gate to available stage types
src/gui/app.rs	Handles noise gate messages and builds stages in the amplifier chain