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diff --git a/image-builder/lib.sh b/image-builder/lib.sh
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+#
+# Copyright (C) 2026 Danilo M. <danix@danix.xyz>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License version 2 as
+# published by the Free Software Foundation.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# Shared helpers for the image builder. Source, do not execute.
+# Sourcing must have no side effects beyond defining functions.
+
+_log()  { echo "[$(date '+%Y-%m-%d %H:%M:%S')] [${LOG_TAG:-image-builder}] $*"; }
+_warn() { echo "[$(date '+%Y-%m-%d %H:%M:%S')] [${LOG_TAG:-image-builder}] WARNING: $*" >&2; }
+_err()  { echo "[$(date '+%Y-%m-%d %H:%M:%S')] [${LOG_TAG:-image-builder}] ERROR: $*" >&2; exit 1; }
+
+# mirror_path MIRROR RELPATH
+# Join a MIRROR (file:///... or http(s)://...) with a relative path.
+mirror_path() {
+    local mirror="$1" rel="$2"
+    printf '%s/%s' "${mirror%/}" "${rel#/}"
+}
+
+# is_local_mirror MIRROR -> 0 if file:// or bare absolute path, else 1
+is_local_mirror() {
+    case "$1" in
+        file://*) return 0 ;;
+        http://*|https://*) return 1 ;;
+        /*) return 0 ;;
+        *) return 1 ;;
+    esac
+}
+
+# strip_scheme URL -> filesystem path for a file:// or bare-local URL
+strip_scheme() {
+    local u="$1"
+    case "$u" in
+        file://*) printf '%s' "${u#file://}" ;;
+        *) printf '%s' "$u" ;;
+    esac
+}
+
+# fetch SRC DEST
+# Copy SRC (a full mirror URL) to DEST. Local mirror -> cp, http -> curl.
+# Returns non-zero on failure; caller decides whether that is fatal.
+fetch() {
+    local src="$1" dest="$2"
+    if is_local_mirror "$src"; then
+        local path; path="$(strip_scheme "$src")"
+        [[ -f "$path" ]] || return 1
+        cp "$path" "$dest"
+    else
+        curl -sSfL --connect-timeout 60 "$src" -o "$dest"
+    fi
+}
+
+# txz_hash DIR
+# Stable hash of the set of Slackware package files (*.t?z) in DIR (names +
+# contents), order
+# independent. Empty/absent dir -> empty string.
+txz_hash() {
+    local dir="$1"
+    [[ -d "$dir" ]] || { printf ''; return 0; }
+    local files; files=$(find "$dir" -maxdepth 1 -name '*.t?z' -type f | sort)
+    [[ -n "$files" ]] || { printf ''; return 0; }
+    # Hash sorted "name  sha256" lines so reordering does not change output.
+    while IFS= read -r f; do
+        printf '%s  %s\n' "$(basename "$f")" "$(sha256sum "$f" | cut -d' ' -f1)"
+    done <<< "$files" | sha256sum | cut -d' ' -f1
+}
+
+# require_mount VERSION
+# Assert the NAS tree for VERSION is mounted (file:// mirror only).
+# x86_64 tree dir is slackware64-${VERSION}.
+require_mount() {
+    local version="$1"
+    is_local_mirror "$MIRROR" || return 0   # http mirror: nothing to check
+    local base; base="$(strip_scheme "$MIRROR")"
+    local dir="${base}/slackware64-${version}"
+    [[ -d "$dir" ]] || _err "NAS tree not mounted: ${dir}"
+}