perf(engine): process the amp chain per-block

Both engine paths looped chain.process(sample) one sample at a time, even
though AmplifierChain::process_block and the Stage::process_block trait method
already existed. Call process_block instead so per-stage work (bypass branch,
stage-list walk) happens once per block rather than once per sample, and so
stages with batched process_block overrides are actually exercised.

Measured on the analog chain: ~31% faster at 1x oversampling, converging to
parity as oversampling rises and real DSP work dominates loop overhead.

Author: OpenSauce

rustortion-core/src/audio/engine.rs

@@ -202,10 +202,7 @@ impl Engine {
     }
 
     fn process_without_upsampling(&mut self, output: &mut [f32]) -> Result<()> {
-        let chain = self.chain.as_mut();
-        for s in output.iter_mut() {
-            *s = chain.process(*s);
-        }
+        self.chain.as_mut().process_block(output);
 
         Ok(())
     }
@@ -215,10 +212,7 @@ impl Engine {
 
         let upsampled = self.samplers.upsample()?;
 
-        let chain = self.chain.as_mut();
-        for s in upsampled.iter_mut() {
-            *s = chain.process(*s);
-        }
+        self.chain.as_mut().process_block(upsampled);
 
         let downsampled = self.samplers.downsample()?;