SWEET_python reports which commit it was installed from

[HughRunyan]

What this is

SWEET_python.__version__ now tells you the git commit this copy was installed from.

Why

The DST endpoints (/sdst, /adst, /cdst) call SWEET_python when a request comes in,
so whatever model code is in the deployed API image is live immediately — there is no
pipeline run and no manifest to look it up in. Nothing logged which commit that was.

Worse, what we recorded could be wrong. WasteMAP's Dockerfiles install SWEET_python with
pip install …@$REF || pip install …@main. If $REF does not exist in this repo, the
image silently gets main instead, while the release manifest goes on recording $REF.

How it resolves

In order, inventing nothing:

1. SWEET_GIT_SHA, which the image build now sets from the ref it actually installed
2. a .git directory beside the package, for a local editable checkout
3. the string "unknown"

VERSION_SOURCE says which of the three it was. A wrong SHA in a log is worse than no
SHA, because a reader believes it.

No version number, on purpose

This does not add a 1.4.2-style version, and setup.py's version="0.1" is left
exactly as it is (now with a comment saying it is decoration).

SWEET_python never ships on its own. It reaches production only inside a Climate TRACE
run or a WasteMAP deploy, and both of those are already identified — by a run_id and a
release tag. A number maintained by hand here would be a second, weaker name for a commit
already recorded automatically, and it would need bumping on every model change by the
same person who has to remember the tag.

Background: VERSIONING.md in
RMI_Climate_TRACE_Waste_Methane#181.

Pairing

Goes with WasteMAP branch wastemap-says-which-model-it-is-running
(RMI/WasteMAP#833), which resolves the ref in the deploy workflow, passes SWEET_GIT_SHA
into both images, and prints it when the API starts.

Acceptance criteria

• SWEET_GIT_SHA wins when set; a blank or whitespace value falls through to the next source
• A local editable checkout reports its own HEAD
• An install that can read neither reports "unknown", never a guess
• version_info() returns the source alongside the value
• Importing the package in a container starts no subprocess — the .git check comes first, and every multiprocessing worker imports this

Definition of done

• Acceptance criteria met
• Tests pass — 247 passed, 6 of them new (tests/test_version_stamp.py)
• Docs updated — the package docstring explains it; setup.py is annotated
• Reviewed & merged

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