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/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