refactor: decompose installer step functions into named sub-steps

The big step functions mixed many concerns in one body, with
essential_services the worst at 442 lines spanning randomness, networking,
SSH, firewall, service discovery, scheduling, and a 177-line filesystem-
branched snapshot block.

I split five of them into thin orchestrators that call named sub-functions,
one per concern:

- essential_services into 11 sub-steps; the snapshot block further splits
  into configure_zfs_snapshots / configure_btrfs_snapshots behind a
  dispatcher.
- prerequisites into bootstrap_pacman_keyring, install_required_software,
  configure_build_environment, configure_package_mirrors.
- developer_workstation into per-category installers.
- boot_ux into one function per hardware-conditional concern (efi perms,
  nvme module, initramfs hook, encrypted autologin, TLP, firmware trim,
  GRUB).
- user_customizations into clone, stow, waybar prune, desktop caches,
  dconf, finalize, directory creation.

Each sub-function body is a verbatim slice of the original (no
re-indentation, so heredoc bodies stay byte-identical). I verified each
extraction by asserting the slices, concatenated in call order, reproduce
the original body exactly, so no line is lost or reordered. bash -n is
clean, the unit suite stays green, and shellcheck gains no real findings.
The SC2329 "never invoked" notes on the new functions are the same
indirect-dispatch false positive every step function already carries.

The flat installers (desktop_environment, supplemental_software) and the
linear preflight_checks are left whole: their length is a sequence of
independent installs or checks, not mixed concerns, so splitting would
scatter a linear read behind indirection.

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