[3dprinter-general] Cura 5.3.1 in experimental

[Christoph Berg]

With the help of AI I disentangled the mess that cura upstream created
after the 5.0 version that we were stuck at. Their "conan" build
system isn't required after all, we just need to copy a bit of cmake
files to 4 of the projects:

https://salsa.debian.org/3dprinting-team/libarcus/-/tree/master/debian/standardprojectsettings?ref_type=heads

Since it's really just these two files I didn't create a new source
package for them.

The version is now 5.3.1 since that's the last version where all the
library packages have proper git tags - after that, they are still
releasing with version number bumps in git, but not tagging anything
but cura/cura-engine.

Also, that's the last version that works without adding a dozen more
packages and waiting for protobuf updates. The LLM's notes say:

### 3. Deferred: 5.14 path

- 5.14 needs ~10-12 new packages and protobuf 6.33.5 (unstable has only
  3.21.12; experimental has 36.2). The 5.14.0 packaging commits/tags for uranium
  and fdm-materials are preserved (`debian/5.14.0-1`, `upstream/5.14.0`) even
  though `master` was reset to the 5.0.0 base.
- Reference versions from the Cura 5.14.0-beta.1 AppImage
  (`/tmp/opencode/opencode/squashfs-root`): arcus 5.11.1, savitar/pyarcus/
  pysavitar 5.12.0, nest2d/pynest2d 5.11.0-alpha.0, mapbox-wagyu 0.5.0, scripta
  1.1.0-alpha.0+b96045, cura-formulae-engine 1.2.1, asio-grpc 3.5.0,
  neargye-semver 0.3.0, curaengine_grpc_definitions 0.4.1, protobuf 6.33.5,
  boost 1.88.0, grpc 1.84.0, zeus_expected 1.4.0, ctre 3.7.2,
  dulcificum 5.11.0-alpha.0, uvula/libUvula 1.1.0.

I might tackle that once we have confirmation that 5.3 works.

Christoph