Feat: add hidden mode and new documentation

src/app/staging.rs

@@ -1,3 +1,5 @@
+//! Staging area for pattern start/stop, mute/solo, and prop changes before commit.
+
 use crate::engine::{PatternChange, SequencerSnapshot};
 use crate::model::bp_label;
 use crate::state::StagedChange;
@@ -5,6 +7,7 @@ use crate::state::StagedChange;
 use super::App;
 
 impl App {
+    /// Toggle a pattern's staged state: unstage if already staged, else stage start or stop.
     pub fn stage_pattern_toggle(
         &mut self,
         bank: usize,