ci: locate maturin-built .so via filesystem, not via import

The previous version of this step did `python -c "import headroom._core"`
to find the wheel's installed `.so` path. That failed in CI:

    ModuleNotFoundError: No module named 'headroom._core'

— exactly the chicken-and-egg this step exists to fix. The editable
install (`pip install -e .`) puts the in-tree `headroom/` source dir
ahead of site-packages on `sys.path`. So `import headroom` finds the
in-tree dir (which doesn't yet have the `.so`), then
`import headroom._core` fails to find the submodule. The symlink we're
about to create is what makes the import work — but we can't import
to discover the symlink target before creating it.

Locate the `.so` via filesystem instead: read site-packages from
`site.getsitepackages()[0]`, glob for `_core.cpython-*.so` under
`<site-packages>/headroom/`, and symlink that into the in-tree dir.
The smoke test (`from headroom._core import DiffCompressor`) runs
*after* the symlink and confirms end-to-end resolution.

Also added `set -euo pipefail` and a sanity check with `ls -la` of the
site-packages dir if the glob comes up empty, so future failures
diagnose themselves.

.github/workflows/ci.yml

@@ -60,11 +60,23 @@ jobs:
       # a wheel + pip-install it instead. Then symlink the `.so` into the
       # in-tree `headroom/` package so the editable install resolves
       # `import headroom._core` past the source dir shadowing site-packages.
+      #
+      # We locate the `.so` via filesystem (not via `import headroom._core`)
+      # because the import would fail at this point — the editable install's
+      # in-tree `headroom/` directory shadows site-packages and doesn't
+      # contain the `.so` yet. That's exactly the problem this step fixes.
       - name: Build Rust extension (headroom._core)
         run: |
+          set -euo pipefail
           maturin build --release -m crates/headroom-py/Cargo.toml --out dist
           pip install --force-reinstall --no-deps dist/headroom_core_py-*.whl
-          SO_FILE=$(python -c "import headroom._core, pathlib; print(pathlib.Path(headroom._core.__file__).resolve())")
+          SITE_PACKAGES=$(python -c "import site; print(site.getsitepackages()[0])")
+          SO_FILE=$(ls "$SITE_PACKAGES"/headroom/_core.cpython-*.so 2>/dev/null | head -1)
+          if [[ -z "$SO_FILE" ]]; then
+            echo "error: could not find _core.cpython-*.so under $SITE_PACKAGES/headroom/" >&2
+            ls -la "$SITE_PACKAGES/headroom/" || true
+            exit 1
+          fi
           ln -sf "$SO_FILE" "headroom/$(basename "$SO_FILE")"
           python -c "from headroom._core import DiffCompressor; print('headroom._core OK:', DiffCompressor)"