filter: sweep the corner continuously; raise the editor floor to 840x620

Cutoff-only re-solve (15.5 vs 56.9 ns/frame) makes the unquantized sweep
affordable, replacing the 2048-step mod quantizer. Live-compute parameters
remain blocked on a shell-architecture ruling.

src/core/instrument/engine/voice.cpp

@@ -97,7 +97,6 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
     // per-frame path, exactly as the amp's velocityGain_ does.
     filterOn_ = p.filter.enabled;
     if (filterOn_) {
-        filterSettings_ = p.filter.settings;
         filterCutoffNorm_ = static_cast<double>(p.filter.settings.cutoffNorm);
         filterModAmount_ = p.filter.modAmount;
         filterKeyTrack_ = p.filter.keyTrack;
@@ -108,6 +107,14 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
         filterEnv_.noteOn();
         filter_.reset();
         updateFilterCutoffBase(note);
+        // The note's ONE full solve — Q, morph and drive are constants for its lifetime, so
+        // every later re-solve is the cheap cutoff-only path. A modulated voice supersedes this
+        // cutoff in tickFilterCutoff on its first frame, before any sample reaches the kernel.
+        instrument::engine::filter::FilterSettings s = p.filter.settings;
+        s.cutoffNorm = filterBaseCutoff_;
+        filter_.prepare(s, filterRate_);
+        filterSolvedCutoff_ = filterBaseCutoff_;
+        filterSolved_ = true;
     }
 
     // Prime the already-sized per-channel shifters with the first window of the actual