lower Vulkan requirement and fix shaderc includes (#7902)

Lower Vulkan version requirement to 1.4.304, which should be the lowest
version supported without requiring code changes. This should also offer
out of the box support on current Linux distros, such as Debian 13 (trixie)
and Ubuntu 25.10 (questing) & 26.04 (resolute).

Also make sure that the shaderc headers are available and added to the
include path on Linux when using system libraries.

Author: notimaginative

lib/vulkan.cmake

@@ -161,7 +161,19 @@ endif()
 # Note that we only rely on the headers and do NOT link against the Vulkan libs!
 
 if(NOT USING_PREBUILT_VULKAN)
-	find_package(Vulkan 1.4.341 REQUIRED)
+	# oldest version that shouldn't require code changes to support
+	find_package(Vulkan 1.4.304 REQUIRED)
+
+	# shaderc ships separately from the Vulkan headers on most Linux distros
+	find_path(SHADERC_INCLUDE_DIR shaderc/shaderc.h)
+
+	if(NOT SHADERC_INCLUDE_DIR)
+		message(FATAL_ERROR
+			"shaderc headers not found. Install them (Debian/Ubuntu: libshaderc-dev) or "
+			"the Vulkan SDK, or configure with -DVULKAN_USE_PRECOMPILED=ON.")
+	endif()
+
+	target_include_directories(Vulkan::Headers SYSTEM INTERFACE "${SHADERC_INCLUDE_DIR}")
 endif()
 
 # prebuilt