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/map/CMakeLists.txt

@@ -15,6 +15,8 @@ reasampler_test(bank_sync LINK bank_sync)
 
 # The state codec is shared with the extension's preset-blob path, so it must link WITHOUT
 # the voice engine: velocity_curve (the curve field) and master_gain (the wire gain cap) only.
+# play_params.h also pulls in filter/'s headers (FilterSettings, MorphLaw) for the v9 filter
+# tail -- plain value types, so no filter symbol is linked and this stays true.
 reasampler_pure_library(component_state_io
     SOURCES component_state_io.cpp
     LINK PUBLIC velocity_curve master_gain)

src/core/instrument/map/component_state_io.cpp

@@ -162,9 +162,14 @@ void readFilterTail(ByteReader& r, InstrumentParams& p) {
     f.settings.driveNorm     = static_cast<float>(bitsToDouble(r.u64()));
     f.settings.morphLaw = (r.u8() != 0) ? engine::filter::MorphLaw::HighNotchLow
                                         : engine::filter::MorphLaw::HighBandLow;
-    f.modAmount = bitsToDouble(r.u64());
-    f.velAmount = bitsToDouble(r.u64());
-    f.keyTrack  = bitsToDouble(r.u64());
+    // Same non-finite-falls-back-to-neutral guard as the v8 master gain above: these three
+    // reach Voice::tickFilterCutoff's clamp compares and a static_cast<int>, both UB on NaN.
+    double modAmount = bitsToDouble(r.u64());
+    double velAmount = bitsToDouble(r.u64());
+    double keyTrack  = bitsToDouble(r.u64());
+    f.modAmount = std::isfinite(modAmount) ? modAmount : 0.0;
+    f.velAmount = std::isfinite(velAmount) ? velAmount : 0.0;
+    f.keyTrack  = std::isfinite(keyTrack) ? keyTrack : 0.0;
     f.env.attackSeconds  = bitsToDouble(r.u64());
     f.env.holdSeconds    = bitsToDouble(r.u64());
     f.env.decaySeconds   = bitsToDouble(r.u64());