extension: kernel-version-toolchain for compiler in artifact version

Add extension that includes compiler identifier (e.g., gcc13.3, clang18.1)
in kernel artifact version string using the new artifact_kernel_version_parts
hook.

Enable with: ENABLE_EXTENSIONS="kernel-version-toolchain"

This ensures kernel cache invalidation when the toolchain changes.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: iav

extensions/kernel-version-toolchain.sh

@@ -0,0 +1,35 @@
+# Add compiler (gcc/clang) identifier to kernel artifact version string.
+# This ensures cache invalidation when the toolchain changes.
+# Enable with: ENABLE_EXTENSIONS="kernel-version-toolchain"
+
+function artifact_kernel_version_parts__add_toolchain() {
+	# Determine compiler binary
+	declare kernel_compiler_bin="${KERNEL_COMPILER}gcc"
+	if [[ "${KERNEL_COMPILER}" == "clang" ]]; then
+		kernel_compiler_bin="clang"
+	fi
+
+	# Get compiler version (major.minor only)
+	declare toolchain_id="unknown"
+	if command -v "${kernel_compiler_bin}" &> /dev/null; then
+		declare full_version
+		full_version="$(${kernel_compiler_bin} -dumpfullversion -dumpversion 2>/dev/null || echo "")"
+		if [[ -n "${full_version}" ]]; then
+			# Extract major.minor, drop patch version
+			declare short_version
+			short_version="$(echo "${full_version}" | cut -d'.' -f1-2)"
+			# Build identifier: gcc13.3 or clang18.1
+			if [[ "${KERNEL_COMPILER}" == "clang" ]]; then
+				toolchain_id="clang${short_version}"
+			else
+				toolchain_id="gcc${short_version}"
+			fi
+		fi
+	fi
+
+	display_alert "Extension: ${EXTENSION}: Adding toolchain to kernel version" "${toolchain_id}" "debug"
+
+	# Add to version parts
+	artifact_version_parts["T"]="${toolchain_id}"
+	artifact_version_part_order+=("T")
+}