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/play_params.h

@@ -96,10 +96,8 @@ struct PitchEnvParams {
 // normalized control positions verbatim rather than a parallel set, so no control range is
 // re-derived here; `filter_params.h` owns every law that maps them to Hz/Q/depth.
 //
-// The three modulation depths all land in that same normalized cutoff domain and sum before a
-// single clamp: `modAmount` scales the per-frame filter envelope, `velAmount` scales the
-// note-on velocity through `velocityCurve`, and `keyTrack` moves cutoff by octaves per octave
-// above the root. All three are zero/neutral by default.
+// The three modulation depths below land in that same normalized cutoff domain and sum
+// before a single clamp; all three are zero/neutral by default.
 struct FilterParams {
     bool enabled = false;
     instrument::engine::filter::FilterSettings settings;
@@ -109,7 +107,10 @@ struct FilterParams {
     AdsrParams env;           // the same staged AHDSR the amp runs; frames
     // Shapes velocity before velAmount scales it. Linear rather than the amp's flat() default
     // because a flat curve under a depth control would make every velocity the same offset;
-    // the no-op at rest is velAmount == 0, not the curve.
+    // the no-op at rest is velAmount == 0, not the curve. NOTE: this default only governs a
+    // FRESH FilterParams — the shared codec's corrupt/truncated-point-list repair
+    // (VelocityCurve::fromPoints, used for both this curve and the amp's) still degrades to
+    // flat() regardless, since that repair has no curve-specific fallback.
     VelocityCurve velocityCurve = VelocityCurve::linear();
 };