feat: per-channel meters, ceiling lamp, and pre-gain drive

Stage 6 begins. Add lock-free Meters (atomics shared audio->GUI) with a
peak-with-decay ballistic, published once per block and gated on the editor
being open. Editor draws a per-channel level + gain-reduction meter panel and
a ceiling lamp fed by the output limiter. Compressor/Limiter expose
gain_reduction_db() for this.

Also add a smoothed per-channel pre-gain applied before each compressor (and
before the All compressor), driving the signal into compression and on into
the limiter for a compressed semi-distortion. Pairs with makeup for full
per-channel input/output gain-staging. Compressor DSP untouched; 16 tests pass.

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

src/params.rs

@@ -55,6 +55,11 @@ pub struct Codename206Params {
 
 #[derive(Params)]
 pub struct CompressorParams {
+    /// Drive into the compressor: scales the signal **before** detection, so it both pushes the
+    /// channel further into compression and feeds the downstream sum/limiter harder. Combined with
+    /// makeup (post-comp), this gives full per-channel input/output gain-staging.
+    #[id = "pregain"]
+    pub pre_gain_db: FloatParam,
     #[id = "detect"]
     pub detection: EnumParam<DetectionMode>,
     #[id = "thresh"]
@@ -129,6 +134,15 @@ impl Default for Codename206Params {
 impl Default for CompressorParams {
     fn default() -> Self {
         Self {
+            pre_gain_db: FloatParam::new(
+                "Pre-gain",
+                0.0,
+                FloatRange::Linear { min: -24.0, max: 36.0 },
+            )
+            .with_smoother(SmoothingStyle::Linear(20.0))
+            .with_unit(" dB")
+            .with_value_to_string(formatters::v2s_f32_rounded(1)),
+
             detection: EnumParam::new("Detection", DetectionMode::Peak),
 
             threshold_db: FloatParam::new(