derive tag debugging

.github/workflows/build.yml

@@ -91,6 +91,26 @@ jobs:
           # De-duplicate
           mapfile -t TARGET_DIRS < <(printf "%s\n" "${TARGET_DIRS[@]}" | awk 'NF && !x[$0]++')
 
+          # Diagnostics: show what we will include in the matrix
+          echo "ALL_LANES (discovered):"
+          for i in "${!ALL_LANES[@]}"; do
+            printf "  [%d] '%s'\n" "$i" "${ALL_LANES[$i]}"
+          done
+
+          echo "TARGET_DIRS (after selection & dedupe):"
+          for i in "${!TARGET_DIRS[@]}"; do
+            # Show non-printable/empty clearly
+            printf "  [%d] '%s' (len=%d)\n" "$i" "${TARGET_DIRS[$i]}" "${#TARGET_DIRS[$i]}"
+          done
+
+          # Fail if any empty entries sneaked in; that would cause ambiguous matrix entries
+          for val in "${TARGET_DIRS[@]}"; do
+            if [[ -z "${val}" ]]; then
+              echo "ERROR: Computed target dirs contains an empty entry. Aborting to avoid ambiguous matrix." >&2
+              exit 1
+            fi
+          done
+
           if [[ ${#TARGET_DIRS[@]} -eq 0 ]]; then
             echo "No lane directories changed. Skipping build."
             echo "should_build=false" >> $GITHUB_OUTPUT