test(ci): sweep every module for dangling imports (t41)

Two defects shipped with a green suite: button.py imported a helper that had
been deleted (no harness ever loaded the file), and the drift sweep only judged
`homeassistant.*`, so a relative import of a name that no longer exists was
invisible to it. Both are now impossible to ship silently.

* tests/test_module_coverage.py loads EVERY *.py file of the package, in a clean
  interpreter, under the stub harness — exhaustive by construction (a new
  platform is covered the moment the file exists; there is no list and no
  special case). It asserts the reported set equals the files on disk, so a
  silently skipped module fails too;
* entity_stublib.load_entity_modules() now also loads button.py (the named gap);
  the sweep is what keeps that list from needing to be complete;
* ha_api_drift gained pass 5: the package's own relative imports. Pure AST,
  no homeassistant required, with its own entry point
  `scan_package_imports()` for the fast suite — it checks the target file exists
  and defines every imported name (submodule imports still resolve), instead of
  trusting that the modules a harness happened to load are the ones that matter;
* the file walk in both AST passes is recursive, and the non-vacuity floors moved
  with it (modules 10 → 20, relative_imports ≥ 100 — a shallow-glob regression
  trips the floor instead of looking green);
* two permanent red proofs, both on a copy of the tree with the reverted
  `from .helpers import discover_adb_host` reference in button.py: the stub-harness
  sweep fails with "FAIL button.py", and the static pass fails with
  "button.py:1: broken-relative-import: … not defined in helpers.py". Manual runs
  confirm the engine's other direction on the real pre-fix tree (c588a7c: exit 1
  with the t40 entity-contract finding) and exit 0 on HEAD.

No Home-Assistant API drift (23 modules, 58 imports, 148 relative imports, 36
call arguments, 6 awaited HA calls). Verified: 334 passed / 2 skipped, 335 / 1
with androidtvremote2, ruff clean.

Note (pre-existing, not introduced here): running a SUBSET of test files in an
order the suite was not written for (e.g. test_init.py alone after
test_standalone_setup.py) fails 3 import-step tests with
"ModuleNotFoundError: No module named 'voluptuous'" — the per-file stub scoping
assumes the suite's order. Verified identical on the pre-t41 tree (60d884b), so
it is left alone here.

tests/test_module_coverage.py

@@ -0,0 +1,198 @@
+"""tests/test_module_coverage.py — nothing in the package may dangle (t41).
+
+Two defects motivated this file, and both shipped with a green suite:
+
+1. ``button.py`` imported a helper that had been deleted. No harness ever loaded
+   that file (``entity_stublib.load_entity_modules`` lists the platforms it
+   knows), so the ImportError would have appeared only in a user's log. The fix
+   is exhaustive by construction: the sweep walks the package for ``*.py`` files
+   in a clean interpreter, under the stub harness, and loads every one of them.
+   A new platform is covered the moment the file exists — there is no list to
+   update and no special case.
+
+2. The drift sweep resolved ``homeassistant.*`` imports only, so it could not
+   see a relative import of a name that no longer exists — the same defect one
+   layer up. ``ha_api_drift.scan_package_imports`` now resolves the package's own
+   imports from the AST, with no Home Assistant and therefore no stub needed.
+
+Each sweep carries its own red proof: a copy of the tree with the reverted
+``discover_adb_host`` reference must fail it, or the check is decoration.
+"""
+
+import shutil
+import subprocess
+import sys
+from pathlib import Path
+
+import ha_api_drift
+import pytest
+from conftest import _SHIELD_DIR
+
+TESTS_DIR = Path(__file__).resolve().parent
+REPO_ROOT = _SHIELD_DIR.parent.parent
+
+#: Loads every module of a package under the stub harness, in a clean process.
+#: ``argv``: 1 = this tests directory, 2 = the package directory to sweep.
+_DRIVER = r"""
+import sys
+from pathlib import Path
+
+tests_dir, package_root = Path(sys.argv[1]), Path(sys.argv[2])
+sys.path.insert(0, str(tests_dir))
+sys.path.insert(0, str(package_root))
+
+import entity_stublib  # noqa: E402
+from conftest import _load_module  # noqa: E402
+
+# conftest eagerly loads a few stdlib-only modules from the REAL tree (it cannot
+# know about a copied one): point the synthetic package at the tree under test
+# and drop what it imported, so every relative import resolves inside the copy.
+sys.modules["nsh"].__path__ = [str(package_root)]
+for name in [n for n in list(sys.modules) if n.startswith("nsh.")]:
+    del sys.modules[name]
+
+entity_stublib.install()
+
+loaded, failures = [], []
+for path in sorted(package_root.rglob("*.py")):
+    if "__pycache__" in path.parts:
+        continue
+    rel = path.relative_to(package_root)
+    if rel.name == "__init__.py":
+        dotted = ".".join(rel.parent.parts)
+        name = "nsh.init" if dotted in ("", ".") else f"nsh.{dotted}"
+    else:
+        name = "nsh." + ".".join(rel.with_suffix("").parts)
+    search = [str(path.parent)] if rel.name == "__init__.py" else None
+    try:
+        _load_module(name, path, search)
+    except Exception as exc:  # noqa: BLE001 - the error IS the finding
+        failures.append(f"{rel}: {type(exc).__name__}: {exc}")
+    else:
+        loaded.append(str(rel))
+
+for rel in loaded:
+    print(f"OK {rel}")
+for failure in failures:
+    print(f"FAIL {failure}")
+print(f"SUMMARY loaded={len(loaded)} failed={len(failures)} total={len(loaded) + len(failures)}")
+sys.exit(1 if failures else 0)
+"""
+
+
+def _package_modules(root: Path) -> set[str]:
+    """Every Python file of a package, as the sweep reports them."""
+    return {
+        str(path.relative_to(root))
+        for path in root.rglob("*.py")
+        if "__pycache__" not in path.parts
+    }
+
+
+def _sweep(root: Path) -> subprocess.CompletedProcess:
+    return subprocess.run(
+        [sys.executable, "-c", _DRIVER, str(TESTS_DIR), str(root)],
+        capture_output=True,
+        text=True,
+        cwd=str(REPO_ROOT),
+        check=False,
+    )
+
+
+def _reverted_copy(tmp_path: Path) -> Path:
+    """A copy of the package where button.py imports the deleted helper again."""
+    target = tmp_path / "nvidia_shield"
+    shutil.copytree(
+        _SHIELD_DIR,
+        target,
+        ignore=shutil.ignore_patterns("__pycache__", "*.json", "*.png"),
+    )
+    button = target / "button.py"
+    button.write_text(
+        "from .helpers import discover_adb_host  # the reverted reference\n"
+        + button.read_text(encoding="utf-8"),
+        encoding="utf-8",
+    )
+    return target
+
+
+def test_every_module_imports_under_the_stub_harness():
+    """The whole package loads in a clean interpreter — no module is skipped."""
+    result = _sweep(_SHIELD_DIR)
+    output = result.stdout + result.stderr
+
+    assert result.returncode == 0, output
+    reported = {
+        line.split(" ", 1)[1]
+        for line in result.stdout.splitlines()
+        if line.startswith(("OK ", "FAIL "))
+    }
+    assert reported == _package_modules(_SHIELD_DIR), "the sweep skipped or invented a file"
+    assert "failed=0" in result.stdout, output
+
+
+def test_the_sweep_fails_on_a_reverted_helper_reference(tmp_path):
+    """Red proof 1: the exact defect that motivated this check.
+
+    button.py imported ``discover_adb_host`` from helpers after that helper was
+    deleted (t41 removed it with the entity path). The sweep must fail loudly.
+    """
+    result = _sweep(_reverted_copy(tmp_path))
+    output = result.stdout + result.stderr
+
+    assert result.returncode != 0, output
+    assert "discover_adb_host" in output
+    assert "FAIL button.py" in result.stdout
+
+
+def test_every_relative_import_resolves():
+    """The same contract, statically: the package's own imports all resolve."""
+    findings = ha_api_drift.scan_package_imports()
+
+    assert findings == [], "\n".join(str(finding) for finding in findings)
+
+
+def test_the_relative_import_pass_is_not_vacuous():
+    """A pass that resolved nothing would be green for the wrong reason."""
+    report = ha_api_drift.Report()
+
+    ha_api_drift._scan_package_imports([_SHIELD_DIR], report)
+
+    assert report.relative_imports >= ha_api_drift.FLOORS["relative_imports"]
+    # and the walk itself reached every module, not only the top level (the
+    # floor sits above the top-level-only count, so a shallow glob trips it)
+    assert len(_package_modules(_SHIELD_DIR)) >= ha_api_drift.FLOORS["modules"]
+
+
+def test_the_static_sweep_fails_on_a_reverted_helper_reference(tmp_path):
+    """Red proof 2: the AST pass sees the same defect, with no Home Assistant."""
+    findings = ha_api_drift.scan_package_imports(_reverted_copy(tmp_path))
+
+    assert [finding.kind for finding in findings] == ["broken-relative-import"]
+    assert "discover_adb_host" in findings[0].detail
+    assert findings[0].path.endswith("button.py")
+
+
+@pytest.mark.parametrize(
+    ("source", "expected"),
+    [
+        # a name nobody defines
+        ("from .helpers import no_such_helper\n", "broken-relative-import"),
+        # a relative import that leaves the package
+        ("from ..sibling import thing\n", "broken-relative-import"),
+        # a submodule import is valid without a binding in __init__
+        ("from .ha_adb_common import adb_client\n", None),
+    ],
+)
+def test_the_pass_judges_the_cases_that_matter(tmp_path, source, expected):
+    package = tmp_path / "nvidia_shield"
+    (package / "ha_adb_common").mkdir(parents=True)
+    (package / "__init__.py").write_text("", encoding="utf-8")
+    (package / "helpers.py").write_text("def entry_host(entry):\n    return ''\n", encoding="utf-8")
+    (package / "ha_adb_common" / "__init__.py").write_text("", encoding="utf-8")
+    (package / "ha_adb_common" / "adb_client.py").write_text("", encoding="utf-8")
+    (package / "module.py").write_text(source, encoding="utf-8")
+
+    findings = ha_api_drift.scan_package_imports(package)
+
+    assert [finding.kind for finding in findings] == ([expected] if expected else [])