fix(build): make GCC-specific warnings conditional and fix narrowing conversion

- Make -Wduplicated-cond, -Wduplicated-branches, and -Wlogical-op
  GCC-only since Clang doesn't support these flags
- Fix narrowing conversion error in test_discrete_field.cpp by
  explicitly casting int to size_t in initializer list

Author: ahojukka5

cmake/CompilerSettings.cmake

@@ -14,6 +14,12 @@ set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wfatal-errors -Werror=format-security")
 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
   add_compile_options(
     -Wnull-dereference    # Warn about potential null pointer dereferences
+  )
+endif()
+
+# GCC-specific warnings (not supported by Clang)
+if(CMAKE_CXX_COMPILER_ID MATCHES "GNU")
+  add_compile_options(
     -Wduplicated-cond     # Warn about duplicated conditions
     -Wduplicated-branches # Warn about duplicated branches
     -Wlogical-op          # Warn about logical operator issues

tests/unit/core/test_discrete_field.cpp

@@ -116,7 +116,7 @@ TEST_CASE("pfc::interpolate() free function works correctly",
 
     // Initialize: field[i] = i
     for (int i = 0; i < 5; i++) {
-      field[{i}] = i;
+      field[{static_cast<size_t>(i)}] = i;
     }
 
     // Test rounding to nearest (std::round behavior)