fix(iso): fall back through ROCm 6.4→6.3→6.2 if repo Release file missing

ROCm 6.4 does not yet publish a Release file for Debian Bookworm, causing
the live-build chroot hook to fail with "does not have a Release file".

Try each version in ROCM_CANDIDATES order; skip to the next if apt-get update
fails (repo unavailable). Exit gracefully if none are available.
Also rename inner 'candidate' variable to 'smi_path' to avoid collision.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/config/hooks/normal/9001-amd-rocm.hook.chroot

@@ -5,13 +5,13 @@
 
 set -e
 
-ROCM_VERSION="6.4"
+# ROCm versions to try in order (newest first). Fall back if a version's
+# Release file is missing from the repo (happens with brand-new releases).
+ROCM_CANDIDATES="6.4 6.3 6.2"
 ROCM_KEYRING="/etc/apt/keyrings/rocm.gpg"
 ROCM_LIST="/etc/apt/sources.list.d/rocm.list"
 APT_UPDATED=0
 
-echo "=== AMD ROCm ${ROCM_VERSION}: adding repository ==="
-
 mkdir -p /etc/apt/keyrings
 
 ensure_tool() {
@@ -51,11 +51,26 @@ if ! wget -qO- "https://repo.radeon.com/rocm/rocm.gpg.key" \
     exit 0
 fi
 
-cat > "${ROCM_LIST}" <<EOF
-deb [arch=amd64 signed-by=${ROCM_KEYRING}] https://repo.radeon.com/rocm/apt/${ROCM_VERSION} bookworm main
+# Try each ROCm version until apt-get update succeeds (repo has a Release file).
+ROCM_VERSION=""
+for candidate in ${ROCM_CANDIDATES}; do
+    cat > "${ROCM_LIST}" <<EOF
+deb [arch=amd64 signed-by=${ROCM_KEYRING}] https://repo.radeon.com/rocm/apt/${candidate} bookworm main
 EOF
+    if apt-get update -qq 2>/dev/null; then
+        ROCM_VERSION="${candidate}"
+        echo "=== AMD ROCm ${ROCM_VERSION}: repository available ==="
+        break
+    fi
+    echo "WARN: ROCm ${candidate} repository not available for bookworm, trying next..."
+    rm -f "${ROCM_LIST}"
+done
 
-apt-get update -qq
+if [ -z "${ROCM_VERSION}" ]; then
+    echo "WARN: no ROCm apt repository available for bookworm — skipping ROCm install"
+    rm -f "${ROCM_KEYRING}"
+    exit 0
+fi
 
 # rocm-smi-lib provides the rocm-smi CLI tool for GPU monitoring
 if apt-get install -y --no-install-recommends rocm-smi-lib 2>/dev/null; then
@@ -63,9 +78,9 @@ if apt-get install -y --no-install-recommends rocm-smi-lib 2>/dev/null; then
     if [ -x /opt/rocm/bin/rocm-smi ]; then
         ln -sf /opt/rocm/bin/rocm-smi /usr/local/bin/rocm-smi
     else
-        candidate="$(find /opt -path '*/bin/rocm-smi' -type f 2>/dev/null | sort | tail -1)"
-        if [ -n "${candidate}" ]; then
-            ln -sf "${candidate}" /usr/local/bin/rocm-smi
+        smi_path="$(find /opt -path '*/bin/rocm-smi' -type f 2>/dev/null | sort | tail -1)"
+        if [ -n "${smi_path}" ]; then
+            ln -sf "${smi_path}" /usr/local/bin/rocm-smi
         fi
     fi
     rocm-smi --version 2>/dev/null || true