Add a script to check for missing extern_c guards

.extern_c_ignore

@@ -0,0 +1,53 @@
+# .extern_c_ignore
+#
+# Headers that have been reviewed and confirmed NOT to need
+# #ifdef __cplusplus / extern "C" guards.
+#
+# Reasons a header might not need the guard:
+#   - Contains only preprocessor macros / #defines
+#   - Contains only typedefs or static inline functions
+#   - Is a C++-only header (not part of the C API)
+#   - Is an internal/generated header not consumed by user code
+#
+# Format:
+#   - One path per line, relative to the repo root
+#   - Blank lines and lines starting with # are ignored
+#   - Paths use forward slashes (works cross-platform)
+#   - Listing a directory ignores everything underneath it
+
+src/common/pico_base_headers/include/pico/assert.h
+src/common/pico_base_headers/include/pico/config.h
+src/common/pico_base_headers/include/pico/error.h
+src/common/pico_base_headers/include/pico/types.h
+src/common/pico_binary_info/include/pico/binary_info.h
+src/common/pico_binary_info/include/pico/binary_info/code.h
+src/common/pico_binary_info/include/pico/binary_info/defs.h
+src/common/pico_sync/include/pico/lock_core.h
+src/common/pico_sync/include/pico/sync.h
+src/common/pico_usb_reset_interface_headers/include/pico/usb_reset_interface.h
+src/host/pico_stdio/include/pico/stdio.h
+src/host/pico_time_adapter/include/pico/time_adapter.h
+src/rp2040/pico_platform/include/pico/platform/cpu_regs.h
+src/rp2350/pico_platform/include/pico/platform/cpu_regs.h
+src/rp2_common/pico_bootrom/include/pico/bootrom/lock.h
+src/rp2_common/pico_bootrom/include/pico/bootrom/sf_table.h
+src/rp2_common/pico_bootrom/include/pico/bootrom_constants.h
+src/rp2_common/pico_cxx_options/include/pico/cxx_options.h
+src/rp2_common/pico_cyw43_arch/include/pico/cyw43_arch/arch_freertos.h
+src/rp2_common/pico_cyw43_arch/include/pico/cyw43_arch/arch_poll.h
+src/rp2_common/pico_cyw43_arch/include/pico/cyw43_arch/arch_threadsafe_background.h
+src/rp2_common/pico_cyw43_driver/include/pico/btstack_chipset_cyw43.h
+src/rp2_common/pico_fix/rp2040_usb_device_enumeration/include/pico/fix/rp2040_usb_device_enumeration.h
+src/rp2_common/pico_int64_ops/include/pico/int64_ops.h
+src/rp2_common/pico_malloc/include/pico/malloc.h
+src/rp2_common/pico_mem_ops/include/pico/mem_ops.h
+src/rp2_common/pico_platform_sections/include/pico/platform/sections.h
+src/rp2_common/pico_stdio/include/pico/stdio/driver.h
+src/rp2_common/pico_stdio_usb/include/pico/stdio_usb/reset_interface.h
+src/rp2_common/pico_thread_local/include/pico/thread_local.h
+src/rp2_common/pico_time_adapter/include/pico/time_adapter.h
+src/rp2_common/pico_usb_reset/include/pico/usb_reset.h
+src/rp2_common/pico_usb_reset/include/pico/usb_reset_config.h
+src/rp2_common/pico_usb_reset/include/pico/usb_reset_tusb.h
+test/pico_test/include/pico/test.h
+test/pico_test/include/pico/test/xrand.h