ci: declare build-system so uv installs shapepipe

The fast-tests workflow runs 'uv sync --frozen --extra test' then pytest
on the GitHub runner (outside the dev image). Without a [build-system]
table, uv resolved shapepipe as a virtual project (uv.lock recorded
source = { virtual = "." }), so 'uv sync' installed only the
dependencies and not shapepipe itself — every test module failed
collection with 'No module named shapepipe'.

Declare the setuptools build backend (already the de-facto backend via
[tool.setuptools]); uv.lock now records source = { editable = "." } and
'uv sync' puts shapepipe on the path. Verified: 'uv sync --dry-run' now
plans the editable install, and the package builds cleanly with
setuptools.build_meta (wheel contains shapepipe/ + entry points).

Author: claude

pyproject.toml

@@ -91,6 +91,13 @@ environments = ["sys_platform == 'linux'"]
 [tool.uv.sources]
 ngmix = { git = "https://github.com/esheldon/ngmix", tag = "v2.4.0" }
 
+[build-system]
+# Declaring a build backend makes shapepipe an installed package (not a uv
+# "virtual" project), so `uv sync` puts it on the path — required for the
+# fast-tests CI job to import shapepipe.
+requires = ["setuptools>=61"]
+build-backend = "setuptools.build_meta"
+
 [tool.setuptools]
 script-files = [
     "scripts/python/update_runs_log_file.py",