**Refactor Nix init into modular scripts and update packaging paths**

* Move the Nix bootstrap from *scripts/init-nix.sh* to *scripts/nix/init.sh* with split-out helpers in *scripts/nix/lib/*
* Update Arch/Debian/Fedora packaging hooks to call */usr/lib/package-manager/nix/init.sh*
* Keep bootstrap behavior the same while improving maintainability and reuse

https://chatgpt.com/share/693c7159-b340-800f-929e-2515eeb0dd03

scripts/nix/lib/install.sh

@@ -0,0 +1,63 @@
+#!/usr/bin/env bash
+
+if [[ -n "${PKGMGR_NIX_INSTALL_SH:-}" ]]; then
+  return 0
+fi
+PKGMGR_NIX_INSTALL_SH=1
+
+# Requires: NIX_INSTALL_URL, NIX_DOWNLOAD_MAX_TIME, NIX_DOWNLOAD_SLEEP_INTERVAL
+
+# Download and run Nix installer with retry
+#   Usage: install_nix_with_retry daemon|no-daemon [run_as_user]
+install_nix_with_retry() {
+  local mode="$1"
+  local run_as="${2:-}"
+  local installer elapsed=0 mode_flag
+
+  case "$mode" in
+    daemon)    mode_flag="--daemon" ;;
+    no-daemon) mode_flag="--no-daemon" ;;
+    *)
+      echo "[init-nix] ERROR: Invalid mode '$mode' (expected 'daemon' or 'no-daemon')."
+      exit 1
+      ;;
+  esac
+
+  installer="$(mktemp -t nix-installer.XXXXXX)"
+  chmod 0644 "$installer"
+
+  echo "[init-nix] Downloading Nix installer from $NIX_INSTALL_URL (max ${NIX_DOWNLOAD_MAX_TIME}s)..."
+
+  while true; do
+    if curl -fL "$NIX_INSTALL_URL" -o "$installer"; then
+      echo "[init-nix] Successfully downloaded installer to $installer"
+      break
+    fi
+
+    elapsed=$((elapsed + NIX_DOWNLOAD_SLEEP_INTERVAL))
+    echo "[init-nix] WARNING: Download failed. Retrying in ${NIX_DOWNLOAD_SLEEP_INTERVAL}s (elapsed ${elapsed}s)..."
+
+    if (( elapsed >= NIX_DOWNLOAD_MAX_TIME )); then
+      echo "[init-nix] ERROR: Giving up after ${elapsed}s trying to download Nix installer."
+      rm -f "$installer"
+      exit 1
+    fi
+
+    sleep "$NIX_DOWNLOAD_SLEEP_INTERVAL"
+  done
+
+  if [[ -n "$run_as" ]]; then
+    chown "$run_as:$run_as" "$installer" 2>/dev/null || true
+    echo "[init-nix] Running installer as user '$run_as' ($mode_flag)..."
+    if command -v sudo >/dev/null 2>&1; then
+      sudo -u "$run_as" bash -lc "sh '$installer' $mode_flag"
+    else
+      su - "$run_as" -c "sh '$installer' $mode_flag"
+    fi
+  else
+    echo "[init-nix] Running installer as current user ($mode_flag)..."
+    sh "$installer" "$mode_flag"
+  fi
+
+  rm -f "$installer"
+}