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).

tests/test_component_state_io.cpp

@@ -11,6 +11,7 @@
 
 #include <cstdio>
 #include <cstring>
+#include <limits>
 #include <string>
 #include <vector>
 
@@ -608,6 +609,30 @@ static void testFilterTailRoundTripsLosslessly() {
         reasampler::instrument::engine::VelocityCurve::flat()));
 }
 
+// A non-finite modAmount/velAmount/keyTrack (a corrupt blob, or any writer that skipped the
+// same guard the v8 master gain already applies) must lift to the neutral default rather than
+// reach Voice::tickFilterCutoff, where both clamp compares are false against NaN and the
+// static_cast<int> is UB. Mirrors testCorruptFieldsFallBackToDefaults' per-field precedent.
+static void testNonFiniteFilterFieldsLiftToTheNeutralDefault() {
+    ComponentState in;
+    in.selectionId = "pad";
+    FilterSeconds& f = in.params.play.filter;
+    f.enabled = true;
+    f.modAmount = std::numeric_limits<double>::quiet_NaN();
+    f.velAmount = std::numeric_limits<double>::infinity();
+    f.keyTrack = -std::numeric_limits<double>::infinity();
+
+    const ComponentState out =
+        deserializeComponentState(serializeComponentState(in), 48000.0);
+    const FilterSeconds& g = out.params.play.filter;
+    const FilterSeconds def;
+    CHECK(g.modAmount == def.modAmount);
+    CHECK(g.velAmount == def.velAmount);
+    CHECK(g.keyTrack == def.keyTrack);
+    // The fallback is per-field, not per-record: the untouched fields still round-trip.
+    CHECK(g.enabled);
+}
+
 // The WRITER emits the CURRENT payload version, and the marker + version sit at the head of
 // the payload — the self-describing property every legacy branch depends on. Asserted
 // against the semantic constants, not literals.
@@ -1080,6 +1105,7 @@ int main() {
     testTruncationDegradesCleanly();
     testV8RecordLiftsToTheOffNeutralFilter();
     testFilterTailRoundTripsLosslessly();
+    testNonFiniteFilterFieldsLiftToTheNeutralDefault();
     if (failures == 0) {
         std::printf("component_state_io_tests: all tests passed\n");
         return 0;