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.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};
         }