feat: add fetch-libstorage.sh to download prebuilt library

Script auto-detects OS/arch, downloads the matching libstorage release
zip from GitHub, and extracts it into the layout expected by CMakeLists.txt.
Also updates .gitignore to exclude build/ and libstorage/ directories.

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

.gitignore

@@ -1,2 +1,4 @@
 cmake-*
+build/
+libstorage/
 .idea

scripts/fetch-libstorage.sh

@@ -0,0 +1,69 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+LIBSTORAGE_VERSION="v0.3.1"
+
+# --- Detect OS ---
+case "$(uname -s)" in
+    Linux)  OS="linux" ;;
+    Darwin) OS="darwin" ;;
+    *)      echo "Error: unsupported OS '$(uname -s)'" >&2; exit 1 ;;
+esac
+
+# --- Detect architecture ---
+case "$(uname -m)" in
+    x86_64)         ARCH="amd64" ;;
+    aarch64|arm64)  ARCH="arm64" ;;
+    *)              echo "Error: unsupported architecture '$(uname -m)'" >&2; exit 1 ;;
+esac
+
+# --- Check dependencies ---
+for cmd in curl unzip; do
+    if ! command -v "$cmd" &>/dev/null; then
+        echo "Error: '$cmd' is required but not found" >&2
+        exit 1
+    fi
+done
+
+# --- Construct URL and destination ---
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+DEST_DIR="${SCRIPT_DIR}/../libstorage"
+ZIP_NAME="libstorage-${OS}-${ARCH}-${LIBSTORAGE_VERSION}.zip"
+URL="https://github.com/logos-storage/logos-storage-nim/releases/download/${LIBSTORAGE_VERSION}/${ZIP_NAME}"
+
+echo "Fetching ${ZIP_NAME} ..."
+
+# --- Download to temp file ---
+TMPFILE="$(mktemp)"
+trap 'rm -f "$TMPFILE"' EXIT
+
+if ! curl -fSL -o "$TMPFILE" "$URL"; then
+    echo "Error: failed to download ${URL}" >&2
+    exit 1
+fi
+
+# --- Prepare destination ---
+rm -rf "$DEST_DIR"
+mkdir -p "${DEST_DIR}/library" "${DEST_DIR}/build"
+
+# --- Extract ---
+TMPDIR_EXTRACT="$(mktemp -d)"
+trap 'rm -f "$TMPFILE"; rm -rf "$TMPDIR_EXTRACT"' EXIT
+
+unzip -q -o "$TMPFILE" -d "$TMPDIR_EXTRACT"
+
+# --- Place files into expected layout ---
+# Determine shared library extension
+case "$OS" in
+    linux)  LIB_EXT="so" ;;
+    darwin) LIB_EXT="dylib" ;;
+esac
+
+cp "${TMPDIR_EXTRACT}/libstorage.h"       "${DEST_DIR}/library/"
+cp "${TMPDIR_EXTRACT}/libstorage.${LIB_EXT}" "${DEST_DIR}/build/"
+
+echo ""
+echo "libstorage ${LIBSTORAGE_VERSION} installed to: ${DEST_DIR}"
+echo ""
+echo "Use with cmake:"
+echo "  cmake -B build -S . -DLOGOS_STORAGE_NIM_ROOT=${DEST_DIR}"