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/limiter.rs

@@ -89,6 +89,12 @@ impl Limiter {
         self.fixed_delay as u32
     }
 
+    /// Current limiter gain reduction in dB (>= 0). `gain` is linear (<= 1); expressed here as a
+    /// positive dB amount for the ceiling lamp / metering.
+    pub fn gain_reduction_db(&self) -> f32 {
+        -20.0 * self.gain.max(1e-9).log10()
+    }
+
     /// Limit one frame in place: `input[ch]` -> `output[ch]`.
     ///
     /// `ceiling` is linear gain (e.g. `util::db_to_gain(ceiling_db)`); `release_coef` comes from a