Build fails with missing libubox20260213 / libblobmsg-json20260213 on 25.12.0

## Build fails with missing libubox20260213 / libblobmsg-json20260213 on 25.12.0 **Describe the bug** When requesting a custom image build for OpenWrt 25.12.0, the build fails because `block-mount` and `mtd` reference `libubox20260213` and `libblobmsg-json20260213`, which are not available in the release build environment. The build server appears to pull snapshot/trunk versions of `block-mount` (`2026.02.15~8d377aa6-r1`) and `mtd` (`mtd-27`) which depend on a newer ABI version of libubox that does not exist in the 25.12.0 release package index. **This bug affects two independent entry points that both use the same ASU backend:** 1. **`luci-app-attendedsysupgrade`** – the ASU client built into the OpenWrt firmware (tested when upgrading from 24.10.5 to 25.12.0) 2. **`firmware-selector.openwrt.org`** – the web-based firmware selector (tested with a manual custom package list for 25.12.0) Both produce the exact same error, confirming the issue is on the **server side**, not in the client. **To Reproduce** via firmware-selector.openwrt.org: 1. Go to https://firmware-selector.openwrt.org 2. Select device: `Linksys WRT1200AC` (target: `mvebu/cortexa9`) 3. Select version: `25.12.0` 4. Add any custom packages that include `block-mount` or `mtd` 5. Request build → build fails via luci-app-attendedsysupgrade: 1. Run OpenWrt 24.10.5 on the same device 2. Open LuCI → System → Attended Sysupgrade 3. Request upgrade to 25.12.0 with existing package list 4. Build fails with the same error **Error message** ``` ERROR: unable to select packages: libblobmsg-json20260213 (no such package): required by: block-mount-2026.02.15~8d377aa6-r1[libblobmsg-json20260213] libubox20260213 (no such package): required by: block-mount-2026.02.15~8d377aa6-r1[libubox20260213] mtd-27[libubox20260213] ``` **Expected behavior** The build server should use the release versions of `block-mount` and `mtd` that match the 25.12.0 release package index, not snapshot/trunk versions with newer ABI dependencies. **Environment** - Device: Linksys WRT1200AC - Target: `mvebu/cortexa9` - Requested version: `25.12.0` - Client: `luci-app-attendedsysupgrade` (v26.005.68630~af8a35c) AND firmware-selector.openwrt.org - ASU server: https://sysupgrade.openwrt.org