**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/README.md

@@ -0,0 +1,53 @@
+# Nix Bootstrap (package-manager)
+
+This directory contains the **Nix initialization and bootstrap logic** used by *package-manager* to ensure the `nix` command is available on supported systems (host machines and CI containers).
+
+It is invoked during package installation (Arch/Debian/Fedora scriptlets) and can also be called manually.
+
+---
+
+## Entry Point
+
+- *scripts/nix/init.sh*  
+  Main bootstrap script. It:
+  - checks whether `nix` is already available
+  - adjusts `PATH` for common Nix locations
+  - installs Nix when missing (daemon install on systemd hosts, single-user in containers)
+  - ensures predictable `nix` availability via symlinks (without overwriting distro-managed paths)
+  - validates that `nix` is usable at the end (CI-safe)
+
+---
+
+## Library Layout
+
+The entry point sources small, focused modules from *scripts/nix/lib/*:
+
+- *config.sh* — configuration defaults (installer URL, retry timing)
+- *detect.sh* — container detection helpers
+- *path.sh* — PATH adjustments and `nix` binary resolution helpers
+- *symlinks.sh* — user/global symlink helpers for stable `nix` discovery
+- *users.sh* — build group/users and container ownership/perms helpers
+- *install.sh* — installer download + retry logic and execution helpers
+
+Each library file includes a simple guard to prevent double-sourcing.
+
+---
+
+## When It Runs
+
+This bootstrap is typically executed automatically:
+
+- Arch: post-install / post-upgrade hook
+- Debian: `postinst`
+- Fedora/RPM: `%post`
+
+
+---
+
+## Notes / Design Goals
+
+- **Cross-distro compatibility:** supports common Linux layouts (including Arch placing `nix` in */usr/sbin*).
+- **Non-destructive behavior:** avoids overwriting distro-managed `nix` binaries.
+- **CI robustness:** retry logic for downloads and a final `nix` availability check.
+- **Container-safe defaults:** single-user install as a dedicated `nix` user when running as root in containers.
+