TST: implement both-way backward compatibility CI checks for both workspace packages

[neutrinoceros]

Close #475
The first commit is temporary.

[neutrinoceros]

I think I may be hitting a bug in uv.
Specifically, here's the current (as of uv 0.8.14) help for uv sync's --no-sources argument

--no-sources
Ignore the tool.uv.sources table when resolving dependencies. Used to lock
against the standards-compliant, publishable package metadata, as opposed to
using any workspace, Git, URL, or local path sources

(emphasis mine)

I would thus expect that

uv sync --package nonos --all-extras --no-sources

results in an env where nonos itself is installed from source, but nonos-cli is installed from PyPI. Instead, I get this:

Resolved 71 packages in 6ms
Installed 22 packages in 82ms
 + cblind==2.3.1
 + cmocean==4.0.3
 + cmyt==2.0.2
 + contourpy==1.3.3
 + cycler==0.12.1
 + fonttools==4.59.1
 + inifix==6.1.1
 + kiwisolver==1.4.9
 + lick==0.7.1
 + loguru==0.7.3
 + matplotlib==3.10.5
 + nonos==0.19.1 (from file:///Users/clm/dev/la-niche/nonos)
 + nonos-cli==0.1.1 (from file:///Users/clm/dev/la-niche/nonos/cli)
 + numpy==2.3.2
 + packaging==25.0
 + pillow==11.3.0
 + pyparsing==3.2.3
 + python-dateutil==2.9.0.post0
 + rlic==0.5.1
 + scipy==1.16.1
 + six==1.17.0
 + tqdm==4.67.1

I don't know if that behavior is expected or not. Admittedly the dependency cycle between both workspace packages may constitute an edge case, either broken or unsupported. Either way, I'll report this upstream later if I cannot figure it out myself in the interval.

I also note that neither --no-install-workspace (skip any workspace members, including the --package argument) nor --no-install-project (which conceptually translates to the infamously missing --only-deps arg from pip install) fit my needs.

[neutrinoceros]

I think this upstream discussion is close to what I'd need: astral-sh/uv#17351