**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/init.sh

@@ -0,0 +1,130 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# shellcheck source=lib/config.sh
+# shellcheck source=lib/detect.sh
+# shellcheck source=lib/path.sh
+# shellcheck source=lib/symlinks.sh
+# shellcheck source=lib/users.sh
+# shellcheck source=lib/install.sh
+
+SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
+
+source "${SCRIPT_DIR}/lib/config.sh"
+source "${SCRIPT_DIR}/lib/detect.sh"
+source "${SCRIPT_DIR}/lib/path.sh"
+source "${SCRIPT_DIR}/lib/symlinks.sh"
+source "${SCRIPT_DIR}/lib/users.sh"
+source "${SCRIPT_DIR}/lib/install.sh"
+
+echo "[init-nix] Starting Nix initialization..."
+
+main() {
+  # Fast path: already available
+  if command -v nix >/dev/null 2>&1; then
+    echo "[init-nix] Nix already available on PATH: $(command -v nix)"
+    ensure_nix_on_path
+
+    if [[ "${EUID:-0}" -eq 0 ]]; then
+      ensure_global_nix_symlinks "$(resolve_nix_bin 2>/dev/null || true)"
+    else
+      ensure_user_nix_symlink "$(resolve_nix_bin 2>/dev/null || true)"
+    fi
+
+    return 0
+  fi
+
+  ensure_nix_on_path
+
+  if command -v nix >/dev/null 2>&1; then
+    echo "[init-nix] Nix found after PATH adjustment: $(command -v nix)"
+    if [[ "${EUID:-0}" -eq 0 ]]; then
+      ensure_global_nix_symlinks "$(resolve_nix_bin 2>/dev/null || true)"
+    else
+      ensure_user_nix_symlink "$(resolve_nix_bin 2>/dev/null || true)"
+    fi
+    return 0
+  fi
+
+  local IN_CONTAINER=0
+  if is_container; then
+    IN_CONTAINER=1
+    echo "[init-nix] Detected container environment."
+  else
+    echo "[init-nix] No container detected."
+  fi
+
+  # -------------------------------------------------------------------------
+  # Container + root: dedicated "nix" user, single-user install
+  # -------------------------------------------------------------------------
+  if [[ "$IN_CONTAINER" -eq 1 && "${EUID:-0}" -eq 0 ]]; then
+    echo "[init-nix] Container + root: installing as 'nix' user (single-user)."
+
+    ensure_nix_build_group
+
+    if ! id nix >/dev/null 2>&1; then
+      echo "[init-nix] Creating user 'nix'..."
+      local BASH_SHELL
+      BASH_SHELL="$(command -v bash || true)"
+      [[ -z "$BASH_SHELL" ]] && BASH_SHELL="/bin/sh"
+      useradd -m -r -g nixbld -s "$BASH_SHELL" nix
+    fi
+
+    ensure_nix_store_dir_for_container_user
+
+    install_nix_with_retry "no-daemon" "nix"
+
+    ensure_nix_on_path
+
+    # Ensure stable global symlink(s) (sudo secure_path friendly)
+    ensure_global_nix_symlinks "/home/nix/.nix-profile/bin/nix"
+
+    # Ensure non-root users can traverse and execute nix user profile
+    ensure_container_profile_perms
+
+  # -------------------------------------------------------------------------
+  # Host (no container)
+  # -------------------------------------------------------------------------
+  else
+    if command -v systemctl >/dev/null 2>&1; then
+      echo "[init-nix] Host with systemd: using multi-user install (--daemon)."
+      if [[ "${EUID:-0}" -eq 0 ]]; then
+        ensure_nix_build_group
+      fi
+      install_nix_with_retry "daemon"
+    else
+      echo "[init-nix] No systemd detected: using single-user install (--no-daemon)."
+      if [[ "${EUID:-0}" -eq 0 ]]; then
+        ensure_nix_build_group
+      fi
+      install_nix_with_retry "no-daemon"
+    fi
+  fi
+
+  # -------------------------------------------------------------------------
+  # After install: PATH + symlink(s)
+  # -------------------------------------------------------------------------
+  ensure_nix_on_path
+
+  local nix_bin_post
+  nix_bin_post="$(resolve_nix_bin 2>/dev/null || true)"
+
+  if [[ "${EUID:-0}" -eq 0 ]]; then
+    ensure_global_nix_symlinks "$nix_bin_post"
+  else
+    ensure_user_nix_symlink "$nix_bin_post"
+  fi
+
+  # Final verification (must succeed for CI)
+  if ! command -v nix >/dev/null 2>&1; then
+    echo "[init-nix] ERROR: nix not found after installation."
+    echo "[init-nix] DEBUG: resolved nix path = ${nix_bin_post:-<empty>}"
+    echo "[init-nix] DEBUG: PATH = $PATH"
+    exit 1
+  fi
+
+  echo "[init-nix] Nix successfully available at: $(command -v nix)"
+  echo "[init-nix] Nix initialization complete."
+}
+
+main "$@"