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.

cmake/reasampler_targets.cmake

@@ -12,6 +12,17 @@ function(reasampler_pure_library name)
     if(ARG_LINK)
         target_link_libraries(${name} ${ARG_LINK})
     endif()
+    # A default-less switch missing an enumerator: MSVC's C4062 is off by its /W1 default;
+    # GCC/Clang's -Wswitch is on by default but only warns without -Werror, and this repo
+    # sets no -Wall/-Werror/-W4/-WX anywhere. Promoted to an error only here, on our own
+    # pure libraries, so a deliberately default-less switch (e.g. isLiveDeckParam,
+    # deck_groups.cpp) is a compile error on every toolchain. NOT C4061 (fires even with
+    # a default: present) — that would light up every defensive switch in the tree.
+    if(MSVC)
+        target_compile_options(${name} PRIVATE /we4062)
+    elseif(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
+        target_compile_options(${name} PRIVATE -Werror=switch)
+    endif()
 endfunction()
 
 # Test naming is exceptionless: target <name>_tests is built from tests/test_<name>.cpp