refactor(install): always use root loop-file storage

Remove secondary-disk discovery and whole-disk ZFS creation from the
installer. New installations now always create a sparse ZFS loop-file
pool from free space on /, while existing vpsmgr pools are still adopted.
The installer no longer scans, selects, formats, or modifies devices such
as /dev/vdb, including virtual ISO/CD devices.

Update storage documentation and keep the existing reclaim, pool sizing,
and ZFS quota behavior.

Co-authored-by: CommandCodeBot <noreply@commandcode.ai>

docs/architecture.md

@@ -154,11 +154,11 @@ The panel daemon runs as the dedicated unprivileged `vps` system user
 
 ## Storage
 
-- The pool is ZFS. On first install, `10-incus.sh` either adopts an existing
-  pool, uses a spare whole-disk block device, or (no spare disk) creates a
-  **sparse loop-file pool** sized to a share of the free space on `/`:
-  80% by default, 90% when ≥ 20 GiB free. The loop file only allocates blocks
-  as the pool actually fills. On very small hosts, cap the ZFS ARC
+- The pool is ZFS. On first install, `10-incus.sh` adopts an existing pool or
+  creates a **sparse loop-file pool** sized to a share of the free space on
+  `/`: 80% by default, 90% when ≥ 20 GiB free. New installations never scan,
+  select, format, or modify secondary disks. The loop file only allocates
+  blocks as the pool actually fills. On very small hosts, cap the ZFS ARC
   (`zfs.arc_max`) so container memory keeps priority over the pool's cache.
 - Containers are ZFS clones of the image: the image's blocks are shared
   (copy-on-write), so a well-provisioned image costs one copy no matter how

docs/configuration.md

@@ -34,7 +34,7 @@ same table; `vps config list` shows the live values with this annotation.
 | `net.ext_if` | operator | re-run `vps install` | external NIC (auto-detected from default route) |
 | `net.ipv6_subnet` | operator | re-run `vps install` | global IPv6 prefix for pass-through, e.g. `2602:fada:6::/64`; empty = disabled (does not remove IPv6 state already applied, see note below) |
 | `net.ipv6_mode` | **fixed at install** | — | IPv6 allocation mode: `none` / `prefix` (/112 blocks) / `pool` (per-container address) |
-| `net.ipv6_pool` | operator | next `vps install` | pool-mode address list (bare global addresses or `/128`); editable via the admin panel's IPv6 Pool page |
+| `net.ipv6_pool` | operator | applied immediately | pool-mode address list (bare global addresses or `/128`); editable via the admin panel's IPv6 Pool page |
 | `incus.image` | operator | next `vps add` / reinstall | container image alias |
 | `incus.image_fallback` | operator | next `vps add` / reinstall | fallback remote image |
 | `incus.pool` | **fixed at install** | — | storage pool |