Promote switch-exhaustiveness diagnostic to an error on pure libraries (MSVC + GCC/Clang)

MSVC's C4062 is off by default and GCC/Clang's -Wswitch only warns without
-Werror; this repo sets no -Wall/-Werror anywhere. /we4062 and -Werror=switch
now cover both, scoped to pure libraries only.

src/core/instrument/ui/deck_groups.cpp

@@ -199,9 +199,10 @@ bool isLiveDeckParam(DeckParam id) {
         case DeckParam::kFilterTrigAttackCurve:
         case DeckParam::kFilterTrigDecayCurve:
             return true;
-        // Listed rather than defaulted so a newly added control is a COMPILE error here (the
-        // -Wswitch gate is GCC/Clang; MSVC's C4062 is off at this project's warning level)
-        // instead of silently defaulting to non-live. Reasons live in the header.
+        // Listed rather than defaulted so a newly added control is a COMPILE error here on
+        // every toolchain — /we4062 on MSVC, -Werror=switch on GCC/Clang, both set on this
+        // library alone in cmake/reasampler_targets.cmake — instead of silently defaulting
+        // to non-live. Reasons live in the header.
         case DeckParam::kPlayMode:
         case DeckParam::kPitchEngine:
         case DeckParam::kTrigLength: