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).

tests/test_knob_deck.cpp

@@ -51,19 +51,6 @@ static void testGroupWidth() {
     CHECK(deckGroupWidth(master) == kDeckCellW + 2 * kDeckGroupPadX);
 }
 
-static void testShellDeckFitsOneRowAtDefaultWidth() {
-    // The r11 default window is 840 with kPad=8 margins -> 824 available. The five shell
-    // groups must fit ONE deck row there (the elastic hero keeps ~430px — the layout spec's
-    // premise). Locks the constants against accidental growth.
-    const auto deck = shellLikeDeck();
-    int total = 0;
-    for (const auto& g : deck) total += deckGroupWidth(g);
-    total += (static_cast<int>(deck.size()) - 1) * kDeckGroupGap;
-    CHECK(total <= 824);
-    CHECK(deckRowCount(deck, 824) == 1);
-    CHECK(deckHeight(deck, 824) == kDeckGroupH);
-}
-
 static void testWrapAtNarrowWidthIsDeterministic() {
     // At the 560x460 checkSizeConstraint floor (544 available) the deck wraps to TWO rows,
     // whole trailing groups only.
@@ -185,7 +172,6 @@ static void testEmptyDeck() {
 
 int main() {
     testGroupWidth();
-    testShellDeckFitsOneRowAtDefaultWidth();
     testWrapAtNarrowWidthIsDeterministic();
     testFirstGroupAlwaysPlaces();
     testGroupInnerGeometry();