Derive the version from git instead of a hardcoded 0.1.0

[rec3141]

Why

version = "0.1.0" is hardcoded in pyproject.toml and has never moved, so every build
of this package reports the same version regardless of what is in it.

That matters because microscape-nf installs this with pip install git+https://…. A
363-sample dataset analysed on 2026-07-23 could not say which code produced it: the run
manifest recorded 0.1.0 and nothing else, and the container that ran it had since been
rebuilt. Three versions of the pipeline were in play with no way to tell them apart.

What this does

setuptools-scm derives the version from git describe. Between tags that yields
0.1.1.dev<n>+g<commit> — the version string carries the commit itself, so it identifies
the source even with no other metadata.

state	version
at tag v0.1.0	0.1.0
n commits later	0.1.1.dev<n>+g<commit>

Tag releases deliberately (git tag -a v0.2.0 -m …); everything between tags increments
on its own.

Three details worth noting:

• version_file generates _version.py at build time (gitignored) so __version__
  is real at runtime rather than reconstructed by the caller.
• fallback_version because pip sometimes clones without tags — that should degrade
  to 0.0.0+unknown, not break the install.
• __init__.py reads _version.py, falls back to importlib.metadata for editable
  checkouts, then to 0.0.0+unknown.

Verified

Wheel builds cleanly and _version.py ships inside it. The derived commit matches what
the running container's direct_url.json independently recorded — so the version string
and the install metadata now agree on the same commit, from two different mechanisms.

Companion change in microscape-nf (run-manifest branch) records both, plus the actual
version of every tool the pipeline runs.