feat(weld): add sidechain test build

Weld will phase-align a bass layer to another layer it hears through the
sidechain. This first build only checks that FL Studio delivers the
sidechain to a CLAP aux port: it passes the main layer through unchanged,
meters the sidechain and offers Layer/Sum/Sum dry listening. No latency.

plugins/weld/ui/src/lib.rs

@@ -0,0 +1,115 @@
+//! Weld editor, shared by the CLAP wrapper and the headless renderer.
+use common_ui::*;
+use egui::{self, FontId, RichText};
+use weld_core::{AnalyzerFrame, Listen, Settings};
+
+pub use common_ui::style;
+pub const WIDTH: u32 = 1040;
+pub const HEIGHT: u32 = 650;
+
+pub struct State {
+    pub analyzer: AnalyzerFrame,
+    /// True while a pointer button is held, so knob drags become one host gesture.
+    pub gesture_active: bool,
+    pub help: String,
+}
+impl Default for State {
+    fn default() -> Self {
+        Self {
+            analyzer: AnalyzerFrame::default(),
+            gesture_active: false,
+            help: DEFAULT_HELP.into(),
+        }
+    }
+}
+const DEFAULT_HELP: &str =
+    "Put Weld on one bass layer and send the other layer to its sidechain input.";
+impl State {
+    pub fn accept_frame(&mut self, frame: AnalyzerFrame) {
+        let old = self.analyzer;
+        self.analyzer = frame;
+        // Meters fall smoothly between frames.
+        self.analyzer.main_db = frame.main_db.max(old.main_db - 3.);
+        self.analyzer.side_db = frame.side_db.max(old.side_db - 3.);
+        self.analyzer.side_peak_db = frame.side_peak_db.max(old.side_peak_db - 3.);
+    }
+}
+fn hint(r: &egui::Response, state: &mut State, text: &str) {
+    if r.hovered() || r.has_focus() {
+        state.help = text.into();
+    }
+}
+
+pub fn show(ui: &mut egui::Ui, settings: &mut Settings, state: &mut State) {
+    backdrop(ui);
+    state.help = DEFAULT_HELP.into();
+    state.gesture_active = ui.input(|i| i.pointer.any_down());
+    ui.add_space(2.);
+    header(ui, "WELD", "LAYER ALIGN", |ui| {
+        let r = ui
+            .selectable_label(settings.bypass, RichText::new("BYPASS").size(13.))
+            .on_hover_text("Pass the main layer through unchanged.");
+        if r.clicked() {
+            settings.bypass = !settings.bypass;
+        }
+    });
+    ui.add_space(6.);
+    let frame = state.analyzer;
+    panel(ui, |ui| {
+        ui.set_width(ui.available_width());
+        ui.set_min_height(380.);
+        ui.vertical(|ui| {
+            caption(ui, "SIDECHAIN");
+            let (text, color) = if frame.side_db > -90. {
+                (format!("sidechain: {:.0} dBFS", frame.side_db), TEXT)
+            } else if frame.side_seen {
+                ("sidechain: silent".to_string(), MUTED)
+            } else {
+                ("no sidechain signal".to_string(), DIM)
+            };
+            ui.label(
+                RichText::new(text)
+                    .font(FontId::new(40., egui::FontFamily::Name("medium".into())))
+                    .color(color),
+            );
+            ui.add_space(12.);
+            meter(ui, "MAIN", frame.main_db, STEEL, 520.);
+            meter(ui, "SIDECHAIN", frame.side_db, ACCENT, 520.);
+            meter(ui, "SIDE PEAK", frame.side_peak_db, FOCUS, 520.);
+            ui.add_space(18.);
+            ui.label(
+                RichText::new(
+                    "FL Studio: on the other layer's mixer track, right-click the route arrow \
+                     to this track and choose \"Sidechain to this track\". Then open Weld's \
+                     wrapper settings and set the sidechain input to that track.",
+                )
+                .size(13.)
+                .color(MUTED),
+            );
+        });
+    });
+    ui.add_space(6.);
+    panel(ui, |ui| {
+        ui.set_width(ui.available_width());
+        ui.horizontal(|ui| {
+            caption(ui, "LISTEN");
+            for (listen, label, help) in [
+                (Listen::Layer, "LAYER", "Output the main layer only. Use this in the mix."),
+                (Listen::Sum, "SUM", "Output main + sidechain layer. Mute the other layer's own send while listening."),
+                (Listen::SumDry, "SUM DRY", "Output the unprocessed main + sidechain layer, to compare with SUM."),
+            ] {
+                let r = ui.selectable_label(settings.listen == listen, RichText::new(label).size(13.));
+                if r.clicked() {
+                    settings.listen = listen;
+                }
+                hint(&r, state, help);
+            }
+            ui.add_space(40.);
+            let d = Settings::default();
+            let r = rotary(ui, "Output", &mut settings.output, -12. ..=12., d.output, " dB", 54., ACCENT);
+            hint(&r, state, "Output trim.");
+        });
+    });
+    ui.separator();
+    ui.label(RichText::new(&state.help).color(MUTED));
+}