bug(26.10): `chisel cut` failing for 26.10

`chisel cut` against ubuntu 26.10 fails for every package on every arch with:

```console
$ chisel cut --root rootfs --release ubunutu-26.10 --ignore=unstable libc6_libs
2026/07/06 22:08:18 Consulting release repository...
2026/07/06 22:08:19 Fetching current ubuntu-26.10 release...
2026/07/06 22:08:19 Processing ubuntu-26.10 release...
2026/07/06 22:08:29 Warning: This release is in the "unstable" maintenance status. See https://documentation.ubuntu.com/chisel/en/latest/reference/chisel-releases/chisel.yaml/#maintenance to be safe
2026/07/06 22:08:29 Selecting slices...
2026/07/06 22:08:29 Fetching ubuntu 26.10 stonking suite details...
2026/07/06 22:08:29 Release date: Mon, 06 Jul 2026 19:14:03 UTC
error: main/binary-arm64/Packages is missing from stonking main component digests
```

chisel only reads sha256 from InRelease. Stonking archives have only sha256 (at least so far).

```console
$ curl -sSL http://ports.ubuntu.com/ubuntu-ports/dists/stonking/InRelease | grep -E '^(MD5Sum|SHA1|SHA256|SHA512):'
SHA512:

$ curl -sSL http://archive.ubuntu.com/ubuntu/dists/resolute/InRelease | grep -E '^(MD5Sum|SHA1|SHA256|SHA512):'
MD5Sum:
SHA1:
SHA256:
```

**fix proposal**

use strongest present acceptable checksum

Reproduce the failure with `chisel cut` against the Ubuntu 26.10 release and inspect how InRelease checksum sections are selected and parsed. The fix is done when a release exposing only SHA512 checksums is accepted by choosing the strongest present acceptable checksum, and the command succeeds across the affected package archives.

Written by the indexing model from the issue text.