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_deck_groups.cpp

@@ -96,7 +96,8 @@ static void testWrappedDeckHeightAtThePinnedEditorWidths() {
     // the remaining four fit the second.
     CHECK(deckRowCount(g, kAvailAtDefaultWidth) == 2);
     CHECK(deckHeight(g, kAvailAtDefaultWidth) == 2 * kDeckGroupH + kDeckRowGap);
-    // At the 560 floor it takes four; FILTER is wider than the row on its own.
+    // At the 560 floor it takes four: FILTER (440px) fits the row alone, but not alongside
+    // PITCH + PITCH ENV (318 + 12 + 440 = 770 > 544), so it wraps to its own row.
     CHECK(deckRowCount(g, kAvailAtMinWidth) == 4);
     CHECK(deckHeight(g, kAvailAtMinWidth) == 4 * kDeckGroupH + 3 * kDeckRowGap);