Broken: checksuming fails, packages not installed

When I try to upgrade to OpenWRT 25 the upgrade fails on checksumming. Not much information is given, so I don't know what is calculated and what is expected.

When I force the upgrade the new partition is bootable, but no packages are installed on the new partition.

Edit 2
When I've tried to debug with `bash -x` the script can't find `/abpp-upgrade/temp/kernel.bin` yet from the recorded script there is no information on the `kernel.bin` being extracted.

Edit 3: It's not the download stage (output after removing `-q` from `wget`).
```
Downloading OpenWrt 25.12.2...
Downloading checksums...
Downloading 'https://downloads.openwrt.org/releases/25.12.2/targets/x86/64//sha256sums'
Connecting to 151.101.130.132:443
Writing to 'sha256sums'
sha256sums 100% |*******************************| 182k 0:00:00 ETA
Download completed (186976 bytes)
Connecting to 151.101.2.132:443
Writing to 'sha256sums.sig'
sha256sums.sig 100% |*******************************| 142 0:00:00 ETA
Download completed (142 bytes)
Verifying checksum signatures...
Checksum signature check failed!
```

Edit 3,5: removing `-v` from `usign` yields ~~just `signature check failed`~~
Correction:
```
Verifying checksum signatures...
Cannot open file '/etc/opkg/keys/b5043e70f9a75cde' for reading
Checksum signature check failed!
```

Edit 4:
The referenced `b5043e70f9a75cde` key is described in the OpenWRT keyring repo as “Public usign key for unattended snapshot builds”.

See https://git.openwrt.org/keyring/tree/usign/b5043e70f9a75cde

Edit 5:
It seems that the `sha256sums.sig` was made with the `b5043e70f9a75cde` key, and it is not available in 24's keyring.

```
root@deathbox:~# usign -F -m sha256sums
b5043e70f9a75cde
```

Edit 6: decided to work it around by manually copying that key into `/etc/opkg/keys/`.

[abpp-dump.log](https://github.com/user-attachments/files/26615046/abpp-dump.log)
[abpp-dump_2026-04-09T23:54:43.script.txt](https://github.com/user-attachments/files/26615471/abpp-dump_2026-04-09T23.54.43.script.txt)

Start by reading the attached abpp-dump_2026-04-09T23.54.43.script.txt and abpp-dump.log, then trace the upgrade script's checksum verification and package-install steps. Reproduce the OpenWrt 25 upgrade and inspect the missing b5043e70f9a75cde key and kernel.bin extraction messages. Done means checksum verification succeeds and packages are installed on the new partition without manual key copying.

Written by the indexing model from the issue text.