ostree-ext: Failed assertion in `Chunking::process_mapping`

When rechunking certain images using `rpm-ostree compose build-chunked-oci`, an assertion in [ostree_ext::chunking::Chunking::process_mapping](https://github.com/bootc-dev/bootc/blob/5ec2c5ce690a01c35cd6cfb43cc03e176ab8e96a/crates/ostree-ext/src/chunking.rs#L423) fails, causing a panic. Here are two image builds where this happens:

* https://github.com/get-aurora-dev/aurora-test/actions/runs/20599380284/job/59161093895?pr=4#step:17:29
* https://github.com/Lumaeris/vedaos/actions/runs/20597343502/job/59154715757#step:3:6735

This only happens with certain images, not with all images, and I'm unsure what exactly triggers it. However, it seems to have something to do with basing the layer plan on a previous build manifest, as this bug was not occurring with the same images when the previous build manifest was not being taken into account. (If someone could figure out a more minimal reproduction of this bug, that would be great.)