fix: ship py.typed so downstream type checkers see the annotations

tests/test_packaging.py

@@ -0,0 +1,45 @@
+"""The package declares itself typed, and keeps declaring it.
+
+``py.typed`` is an empty file whose only job is to exist. Nothing in this
+repository breaks if it is deleted, every test still passes, and mypy still
+runs clean here, because the annotations are all present either way. What
+breaks is downstream: PEP 561 says a type checker must ignore a dependency's
+inline annotations unless the package ships this marker, so without it every
+consumer of plumbline sees ``Any`` and never finds out why.
+
+That makes it the kind of file that gets lost in a refactor and is not missed
+for a year. Hence a test.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import plumbline
+
+
+def test_the_package_ships_a_py_typed_marker() -> None:
+    package_root = Path(plumbline.__file__).parent
+
+    assert (package_root / "py.typed").is_file(), (
+        "py.typed is missing. Without it, PEP 561 requires type checkers to "
+        "ignore this package's annotations, so downstream users silently get "
+        "Any for everything plumbline exposes."
+    )
+
+
+def test_the_marker_is_empty_as_pep_561_intends() -> None:
+    """The file is a flag, not a config. Content here would be a mistake."""
+    marker = Path(plumbline.__file__).parent / "py.typed"
+
+    assert marker.read_text(encoding="utf-8").strip() == ""
+
+
+def test_the_version_is_a_release_version_not_a_placeholder() -> None:
+    """Guards the mismatch that shipped in v0.1.0.
+
+    The release was tagged v0.1.0 while the package still called itself
+    0.1.0.dev0, and the test that should have caught it asserted a substring.
+    """
+    assert plumbline.__version__ == "0.1.0"
+    assert "dev" not in plumbline.__version__