feat: live operating-point fill + -6 dB refs on the gain curve

Add a per-channel input_level meter (post pre-gain, peak-with-decay) and use it
to shade the gain curve: a translucent fill under the curve from the floor up to
the current input, its right edge riding the curve (width = input, height =
output), plus a dot at the operating point. Add -6 dBFS reference lines on both
axes with a tick label.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/editor/mod.rs

@@ -84,7 +84,7 @@ pub(crate) fn create(params: Arc<Codename206Params>, meters: Arc<Meters>) -> Opt
                         // Gain curve (left, square) beside the scrolling plot (right, fills the rest).
                         let selected = state.plot.selected;
                         ui.horizontal_top(|ui| {
-                            ui.vertical(|ui| gain_curve::draw(ui, &params, selected));
+                            ui.vertical(|ui| gain_curve::draw(ui, &params, selected, &meters));
                             ui.vertical(|ui| plot::draw(ui, &meters, &mut state.plot));
                         });
                         ui.separator();