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/ui/CMakeLists.txt

@@ -44,8 +44,11 @@ reasampler_test(envelope_edit LINK envelope_edit)
 reasampler_pure_library(knob_deck SOURCES knob_deck.cpp LINK PUBLIC editor_geometry)
 reasampler_test(knob_deck LINK knob_deck)
 
-# The deck's group COMPOSITION, split from its layout: knob_deck stays engine-free, while this
-# names the controls and so reads PlayMode (velocity_curve comes along with play_params.h).
+# The deck's group COMPOSITION, split from its layout: knob_deck stays engine-free (see
+# core/instrument/CLAUDE.md's deck_groups entry for why this module, not knob_deck, reads
+# PlayMode). velocity_curve is the filter's own curve field; peaks is play_params.h's
+# AudioSample dependency. play_params.h also drags in filter/'s headers (FilterSettings,
+# MorphLaw) for the v9 filter tail -- plain value types, no filter symbol linked.
 reasampler_pure_library(deck_groups
     SOURCES deck_groups.cpp
     LINK PUBLIC knob_deck velocity_curve peaks)

src/core/instrument/ui/deck_groups.cpp

@@ -72,7 +72,7 @@ std::vector<DeckGroupDesc> sampleDeckGroups(PlayMode playMode) {
                            id(DeckParam::kRelease)};
         } else {
             // Trigger, time-ordered left-to-right (Fade In / Length % / Fade Out — matches
-            // the drawn envelope), plus the two reserved blanks that hold the Gate width.
+            // the drawn envelope), plus two blanks (see knob_deck.h's blank-cell contract).
             amp.cellIds = {id(DeckParam::kTrigFadeIn), id(DeckParam::kTrigLength),
                            id(DeckParam::kTrigFadeOut), -1, -1};
         }

src/core/instrument/ui/deck_groups.h

@@ -68,9 +68,8 @@ enum DeckGroupId {
 };
 
 // The deck's groups, left to right, in SIGNAL-FLOW order: pitch -> filter -> amp, then the
-// two instance-wide groups. `playMode` picks the AMP group's face; its width is
-// mode-independent (Trigger leaves two blank cells) so a mode flip never reflows the
-// neighbouring groups.
+// two instance-wide groups. `playMode` picks the AMP group's face, via knob_deck's blank-cell
+// reservation (knob_deck.h) so a mode flip never reflows the neighbouring groups.
 std::vector<DeckGroupDesc> sampleDeckGroups(PlayMode playMode);
 
 // The deck's BIPOLAR knob law: 0.5 of the knob's travel is zero depth, the ends are -1 and