fix: guard filter tail NaNs, skip static-filter envelope work, pin stereo filter path, correct stale comments

Codec fallback for non-finite filter fields, a modAmount==0 early-out in tickFilterCutoff,
new stereo render tests for the dual-mono mirror, and comment/test accuracy fixes flagged
in review (stale deck-width claims, restated invariants, drifted CMake link comments).

src/core/instrument/engine/voice.cpp

@@ -92,8 +92,8 @@ void Voice::start(int note, int velocity, const SampleData& sample, bool declick
     pitchEnv_.configure(p.pitchEnv);
     pitchEnv_.noteOn();
 
-    // Filter: fresh integrators per note (prepare() preserves state on purpose, so a note-on
-    // is the one place that must clear it). Velocity maps through the curve once here, off the
+    // Filter: reset() clears integrator state for the new note (prepare() preserves it —
+    // voice_filter.h / filter/CLAUDE.md). Velocity maps through the curve once here, off the
     // per-frame path, exactly as the amp's velocityGain_ does.
     filterOn_ = p.filter.enabled;
     if (filterOn_) {