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/dsp/compressor.rs

@@ -152,6 +152,13 @@ impl Compressor {
         self.fixed_delay as u32
     }
 
+    /// Current gain reduction being applied, in dB (>= 0), excluding makeup. For metering.
+    /// This is the smoothed detector output `yl`, so it tracks the visible needle, not the
+    /// instantaneous static curve.
+    pub fn gain_reduction_db(&self) -> f32 {
+        self.yl
+    }
+
     /// Process one sample frame in place: `input[ch]` -> `output[ch]`.
     ///
     /// `input` and `output` are short stack slices (one value per channel), so this